This is the true problem in RyoSaeba and KaoriMakimura's relationship in City Hunter, as he's perfectly aware of the numerous enemies he has as a sweeper, and said enemies had already targeted Kaori during the course of the series since they usually can't beat him one on one.
At one point during the second season of Code Geass some of Lelouch's flunkies (rightfully) think he is doing this with Shirley. When a body double mix up at his school leads to problems maintaining his cover one points out that it could all be solved simply by getting Shirley involved. Lelouch is reluctant, however, as being near him is a surefire way to get killed like his half-sister Euphemia and he had already mucked up her life horribly, both by tampering extensively with her memories and accidentally killing her father. Unfortunately, she dies anyway soon afterward.
Surprisingly, this one works! And thanks to Kallen working out the details of Zero Requiem, she didn't even end up hating him for it.
The entirety of Lelouch and Kallen's character dynamic leans into this as the series moves on, as Kallen slowly starts having feelings for Lelouch, both as himself and as Zero. Lelouch's responses basically amount to finding increasingly creative ways to try and make her disgusted with him, in spite of how he goes out of his way to protect her in other cases.
This is also the main reason he makes up the masked persona of Zero to lead the rebellion: he doesn't want Nunally to be in any danger, and she would be if word got out that Lelouch was the leader of the rebellion.
The Gokusen anime tries a slightly less orthodox method in this. Kumiko/Yankumi tries to live a double life as a teacher and as an heiress to a Yakuza clan. She tries VERY hard to not get her lives to overlap one another, but no matter how hard she tries, if she has to fight any bad gangsters/thugs chances are at least ONE of her students will be involved whether they like it or not. But to be fair this was also to protect her teaching job; while it has been endangered, she never actually lost her job in the anime.
Mahou Sensei Negima! uses this one when Negi tries to convince his True Companions that it would be a bad idea for them to follow him into the Magic World. Cue them all saying that they wouldn't care less and say that they're going with him anyway, for his own safety. This backfires entirely as everyone takes it as evidence that they need to go with him.
This is actually portrayed as a character flaw of Negi's: he constantly wants to keep his friends out of harm's way and do everything dangerous himself, ignoring the fact that he probably would be dead by now if his friends weren't around to help him.
In Mai Hime, Mai tries to push Yuuichi away because the defeat of a Hime leads to the death of her most important person and she had just watched her brother Takumi die after the defeat of his roommate and girlfriend Akira. She fails and Yuuichi dies after his Unlucky Childhood Friend Shiho is defeated.
Melody of Oblivion has a Melos warrior using this excuse on a woman even after he has already kissed her 73 times.
In Rurouni Kenshin, Kenshin leaves to Tokyo to face Shishio on his own and not put the lives of his girlfriend Kaoru and the rest of his True Companions at risk. It doesn't work, they join him anyway
Yuriko, Sakura, and Aya-chan in Weiss Kreuz get this treatment, via the main characters simply disappearing and never seeing them again - for good reason, since the ones who don't get the It's Not You, It's My Enemies treatment (such as Ouka and, in Weiss KreuzGluhen, Asami) always end up dead.
Yu Yu Hakusho played this trope straight as the reason why Yusuke Urameshi keeps Keiko in the dark about his life as a spirit detective is out of fear that if she knows it then it will only be a matter of time until she is in danger. Naturally later on in the 2nd arc it fails miserably and she calls him out on this but to be fair at least she wasn't killed off at the end of that arc.
The storyline Superman: Ending Battle featured Manchester Black learning Clark's secret. To get his revenge on Superman, he uses his mental powers to command every villain in the whole world to kill anyone who has ever shown Clark Kent one iota of kindness. The hit-list includes Lex Luthor, the Daily Planet staff, his parents, his first grade teacher, his old football coach...
X-23 provides a non-romantic version with her parting from her Aunt Debbie and cousin Megan. Unlike most she gives them this speech along with directions to a money stash and instructions on how best to go underground... after their house was blown up and they were nearly tortured to death to punish her.
"How... how will you find us?
"I said, how will you find us?"
This is the reason Batman tries to avoid getting serious with anyone, why he has so few friends, and why he's a Jerk Ass to a majority of his sidekick squad. It's not that he wants to be, he just wants to protect them, because the things hisRogues Gallery does are better not seen by anyone, really. Plus, he doesn't want anyone else growing up like him, bitter and angry at the world, and he's trying to avoid causing anyone pain from when he inevitably falls in combat or finally snaps. Batman's world is not happy.
Heck, in a way, just about ANY given costumed super hero with a secret identity can be quite susceptible to this. (Even characters who don't have a secret identity are not entirely immune from this as well.)
An example for the above statement would be Benjamin Grimm, AKA The Ever-Lovin' Blue-Eyed Thing, who left his fiancee at the altar because of this - he saw Spidey, Daredevil, Banner and Namor sitting together talking at the wedding, and realized that all four of them had lost the women they loved.
In the Ultimate Spider-Man comic book, Peter breaks up with MJ to "keep her safe". It doesn't work, and MJ is still thrown into dangerous situations, so they get back together. Yup, we thought it was stupid, too.
She split up with him, first, for being Spider-Man and therefore incredibly heartbreaking and high-maintenance.
The only person to survive prolonged exposure to him is his mate Chaz. Which makes the fate of Chaz's young sidekick namesake in The Movie particularly unfortunate....
Defied in New Avengers. Both Doctor Strange and Daredevil attempt to refuse an offer of membership because they have enemies, people are out to get them, etc. Luke Cage has to remind them that they're all heroes and they all have enemies.
The first Spider-Man film: Peter gives Mary-Jane the Just Friends excuse when she finally tells him she loves him, after nearly losing her to a Sadistic Choice. (And let's not forget he did lose a love interest in an almost identical scenario in the comic books.) All three movies nonetheless climax with villains luring Spider-Man by holding Mary-Jane hostage, whether they're together or not.
In The Amazing Spider-Man, Peter tries to end his relationship with Gwen this way, but ultimately changes his mind.
The ending of most Batman romantic subplots. In the original Tim Burton films, though, he took a painfully realistic look on this trope, though technically only one relationship ended this way (unmarked spoilers, for Batman fans might typically view this all as It Was His Sled):
Bruce had originally viewed this as his issue with Vicki, until he told her. Of course, this was broken up by The Joker coming in. Vicki ended up dumping him because she couldn't handle it. This ends up haunting him for the rest of the Burton-Shumacher quadrilogy.
In Batman Returns, his issue had been the way he ended with Vicki, leading him to try to get over it with Selina Kyle. Once again, he is the one dumped, as Selina is Ax Crazy by the end of the movie and attempts murder-suicide with her ex-boss, which also caused a Tear Jerker.
Catwoman: Bruce... I would love to live with you in your castle... forever just like in a fairy tale... (scratches Batman's face) I just couldn't live with myself, so don't pretend this is a happy ending!
It's Not You, It's My Enemies only presumably worked with Chase Meridan. After the whole Riddler thing, it seems that she and Batman are going to still be together, but for some reason, the next movie had Batman dating someone else. We can assume that she either got bored with him (and given the character, this is more likely), or Bruce pulled this card.
Joshua in Animal Kingdom is compelled to dump his girlfriend after his criminal family inform him she knows too much about their dealings. Even this doesn't stop Joshua's paranoid Uncle Pope drugging and strangling her.
In the Iron Man movies, Tony Stark subverts this and in doing so shows why it's not necessarily a bad thing. He reveals his identity as Iron Man to the world, and makes no particular secret of his love for Pepper Potts either (though it's not clear if he himself 100% realizes how deep it is)...and what ensues in Iron Man 2 shows that secret identities are not necessarily bad ideas.
Played with in DC Nation with the Original Characters. Green Shield only kept her heroing life a secret until she could figure out how to come out to her folks...only for her kid sister to run off with one of the team's enemies. Travis Gray, and the Zukov siblings never bothered - the siblings only care for each other for the most part, and Travis was living on the streets. Fauna keeps hers because the activist movements she works with and her strictly-pacifist Commune would certainly condemn her choice, and she doesn't want them "paying for" her private decision.
In In Flight, Shirou leaves Rin and Saber behind in England when he runs away before the Mage's Organization does something horrible to him due to this trope, especially as they rely on and belong to said organization.
Harry Potter gives Ginny this speech in The Half-Blood Prince, right after they get together. Ginny, always rather Genre Savvy, doesn't buy it for a second. Rather than react with surprise or tears, she says she understands what he's doing and patiently lets him get on with it. Her later actions in Deathly Hallows indicate that as far as she's concerned, they're still a couple. Harry, amusingly, has absolutely no way of dealing with this and gives in, causing Ron to give Harry the If You Ever Do Anything to Hurt Her... speech when he has trouble letting go. Of course, it's not as though Ginny isn't fighting Voldemort almost as hard as Harry himself.
In Stephen Marley's Chia Black Dragon series, Chia (an immortal lesbian Black Magician Girl) knows that her nemesis, Nyak, will kill any woman she loves, which discourages her from pursuing relationships. Sometimes she does anyway, and the other girl is invariably killed by Nyak. Chia doesn't just have one past girlfriend Stuffed into the Fridge, she has dozens.
By the fourth book in the Wheel of Time series, Rand has three love interests. He is the Chosen One in a Crapsack World. He does not handle being the chosen one well in general, and what would happen to his love interests if his enemies find out about them is one of his many worries. The three of them handle it with varying levels of grace, but they're considering the possibility of polygamy. It helps that one of them is from a culture where that is acceptable.
The inspiration for his behavior was Lan's treatment of Nynaeve.
Rand also takes a similar approach towards his father, Tam al'Thor, and his hometown, the Two Rivers. He figures that the less his enemies know about his attachment to them, especially his adoptive father, the safer they will be from the possibility of attack.
In the Night Huntress books, this is why Cat leaves Bones when the government vampire hunters are closing in, reasoning that they will be safe without the extra attention.
At the end of Changes Harry sends his daughter to be placed in a foster home without him knowing where so that his enemies can't find her. In Ghost Story, we find out the person he entrusts this to does one better and puts her in a house guarded by archangels. Michael's house, specifically.
A version of this is done in New Moon (the second book of Stephenie Meyer's Twilight saga), when Edward leaves Bella, telling her she'll never see him again, in order to keep her safe. Not played exactly straight because he's not protecting her from his enemies as much as he's protecting her fromhis own insatiable thirst for her blood. In actuality he left her unprotected from his enemies.
In Griffins Daughter, Ashinji won't cop to his feelings for half-elf Jelena, partly because of their vastly different social standings (he's elf nobility, she's basically a second class-citizen), but mostly because he fears his sadistic brother, Sadaiyo will hurt her just to spite him. (He's right)
In The X-Files, Mulder tries to use this reasoning on Scully. Needless to say, it backfires.
This is also given as the reason why Mulder is not with Scully and William in season 9.
Supernatural used this trope in "Provenance". Sam tells Sarah they can't be together, which is wise since everyone he sleeps with dies. Later he does sleep with a woman in "Heart". She almost immediately keels over.
Crichton uses this in Farscape, without actually telling Aeryn that's why he's virtually ignoring her. She gets angsty, he starts doing the local equivalent of sniffing pixie dust, and the bad guy KNOWS ANYWAY because it's frelling obvious especially considering she's pregnant at the time. Eventually he lets her in on his fears and they quietly make out after loudly saying "So it's over, then."
This is the entire romantic plot tumor line of approximately the first 7 seasons of Smallville
Inverted in season nine, where Lois gives this speech to Clark, about why it was logical that The Blur didn't tell her his identity, based entirely on this trope. Amusing because Clark was just about to tell her he was the Blur.
Most notably, his ex-fiancée Sam McCall, whose getting shot prompted Jason to leave her for her own protection. Many Jason/Sam shippers still aren't happy.
Kreegan, the protagonist of Touching Evil takes this to the extreme towards the end of the series. After the criminals he is investigating attack his family, he arranges for them to be put into Witness Protection to keep them safe- and makes sure he doesn't know where they've been sent. One of the few examples where the couple never see each other again.
In Being Human, neurotic werewolf George falls in love with his coworker Nina, but struggles with how much to tell her about his condition, and when. He tries to dump her several times, with nebulous claims of "darkness" and "a bad side of me", but she sticks with him.
In Bones Booth breaks up with Camille after the latter was poisoned by a serial killer they were on the trail of, believing their line of work to be too high-risk to be caring for one another at that level.
Played with in Misfits. At Nathan's urging, Curtis says this to Sam. They do break up—but only because Sam recognized the line from Spider-Man and got angry.
Kyle does this to Amanda in the third season of Kyle XY.
Inverted in Psych. After being kidnapped by Yin, Abigail breaks up with Shawn for good because she has dreams of her own she can't achieve if she's dead.
Jack Bauer's reason for not telling anyone other than a select few people that he was alive following the events of the fourth season of 24. Most importantly, Audrey Raines.
Stefan from The Vampire Diaries distances himself from Elena in order to protect her and her friends and family.
Grimm: Nick's aunt Marie tells Nick that he should break up with Juliet because of this trope, and he considers it but doesn't do it. When his best friend (who is part of The Masquerade but should be his enemy) gets beaten up for being his friend, Nick tells him he won't ask him for help anymore, but Monroe tells him he can ask for as much help as he wants. His mother later tells him that he shouldn't live like she did, being afraid of getting to close to people in fear of them getting hurt.
Pretty much the main plot of Star Fox Command revolved around the fact that, before the game began, Fox and Krystal had fallen in love, just before Fox kicks her out of the team and breaks up with her because life as a Star Fox member is so dangerous. Most of the multiple endings wrap this up in some way, sometimes happily, sometimes not.
It also repeats minorly with Amanda (Slippy's Girlfriend) and Lucy, Peppy's daughter.
Between Metal Gear Solid 2 and Metal Gear Solid 4, Rose fakes a miscarriage and a marriage to Roy Campbell to protect herself and her son from the Patriots, who would use them against Raiden. Of course, their relationship was already falling apart before any of this happened, but they worked it out in the end.
Soul Calibur: Kilik knows that his longtime traveling companion Xianghua has romantic feelings for him, but his quest to destroy Soul Edge prevents him from wanting to get too close to her (Kilik's first love, Xianghua's older sister was killed by him while he was under possession of the Evil Seed). In Xianghua's SCIV ending, he explicitly tells this to her as he grabs hold of the cursed sword. She's not listening, no matter what.
In Vampire: The Masquerade - Bloodlines the player has the option to do this to Heather the pet ghoul, after it becomes obvious she's being watched by enemies who have reason to be watching you.
If you send her away, she lives. If you don't, she makes you some armor...and is promptly murdered.
In Baldur's Gate 2, trying to romance drow priestess Viconia will end with this, as she's targeted by Lolth's Handmaidens and doesn't want to drag the protagonist into her mess. However in the expansion she will be amenable to starting over - she realizes that between having your soul ripped out and reclaimed, going to hell, climbing the path to godhood and having to put down your god-blooded half-siblings along the way, the Handmaidens are a minor trifle. Besides, marrying a god...
In Rip and Teri, after catching one of his old enemies attempting to seduce Teri, the woman he loves, in an effort to get to him, secret agent Rip Dustin mentally prepares himself to tell her that he hates her in order to drive her away and ensure her safety from his many enemies - but he's unable to bring himself do so, and can only tell her that "I'll always love you." before leaving her. Needless to say, this doesn't have the driving-away effect he was hoping for.
When the two are reunited a few strips later, Rip attempts to justify his actions with this trope, only for Teri to point out that it wasn't exactly working; she was in just as much danger as if they'd been together because their enemies still tried to use her to get to him - and in fact, because he wasn't around to protect her, she was out of her depth and therefore in an even more dangerous position.
In Sluggy Freelance Torg has put any potential romance with Zoe on hold for years (even calling her a "boring, dumpy chick" once) all in order to keep her safe from his psychotic stalker Oasis.
This was further encouraged when Torg's Arch-Enemy (one of them, anyway) Lord Horribus killed an Alternate Universe version of Zoe that Torg had been dating.
Mel: You know how in almost every superhero movie, the hero breaks up with his girlfriend to protect her? And then she tells him, he's worth the risk? Well... you're not.
Vaarsuvius in The Order of the Stick #679. V's mate has just filed for a divorce, and he chooses not to contest it despite wanting to. And he has a point; though his mate has her own reasons, Vaarsuvius knows that as the quest to save the world continues, he's bound to make more enemies. Enemies that might try and attack Inkyrius and his adopted children, as the Black Dragon did. And curse you, ambiguous gender.
Justice League Unlimited: Diana is flirting with Batman during a stakeout, and he gives a few reasons why they shouldn't. The third reason is the standard superhero wave-off of his enemies being a threat to anyone he's in a relationship with... which Diana answers by casually crushing the head of a stone gargoyle in her hand.
Inverted on Danny Phantom: Valerie, the "villainous" love interest, decides not to pursue her relationship with Danny so she can hunt his Secret Identity: "There's something important I have to do right now, and I don't want you to get hurt because of it." Hey, you can't please everyone. Of course, the irony here is that he's the enemy and already knew her secret identity, making the whole thing rather sad.
Ironically it seems his choice is made when someone who knows him and the original Batman tells him not to distance himself from people who love him - so as to have a better life than the old man (Bruce Wayne).
Avatar The Last Airbender: After his Heel Face Turn in the third season, Zuko leaves behind his girlfriend Mai. As he explains to Sokka in "The Boiling Rock," "Everyone thinks I'm a traitor. I couldn't drag her into it." She is not happy about this and proves how wrongly he underestimated her in spectacular style.