A common observation in fiction: Once you get a girlfriend, other girls just come naturally.
The lead male character may be nice, kind, and fairly attractive. He may adopt stray kittens, help old ladies across the street, do charity work in Third World countries. It doesn't matter, because he's never, ever going to be able to find a girlfriend....at least, not until a girl swoops down out of nowhere and attaches herself limpet-like to his arm.
Suddenly he has an embarrassment of riches. Girls who wouldn't even look at him before are now beating down his door. Old acquaintances he can't even remember will appear and claim to have loved him for years. He can walk down the street and have new characters pop up claiming Love at First Sight.
This can also happen even when the girl is still only a Love Interest, which makes it even harder for the guy to beat off the other girls and convince her that she's his One True Love (which she is). It also makes it harder for him to beat off in general, but that's another story entirely.
As a part of the wish-fulfillment element that defines the Harem Anime genre, the Magnetic Girlfriend will not be jealous of the other girls that enter the equation. This is because she is either too naive to notice their affections, so nice that she would rather be their friend than their rival, or that she trusts her boyfriend to not cheat and so doesn't feel threatened by the extra girls. Oddly enough, her upbeat and kind personality is usually enough to win over their friendship, but the race for the guy is most definitely still on.
Note that if the Magnetic Girlfriend is somehow detached, her effect will often wear off. If the magnetic effect is unnecessary, the guy is a natural Chick Magnet.
Compare Operation: Jealousy where the same 'attraction of the unavailable' is at work.
Truth in Television. Sometimes, the experience of getting a girlfriend gives men newfound confidence that makes them more attractive to other women. Sometimes, the fact that a man isn't on the hunt anymore makes them more relaxed and natural and thus less awkward. Finally, the fact that he has a girlfriend signals to other women that there is something desirable about him, an effect which in Real Life is called "pre-selection" or "social proof." There's also a belief that after having sex, a man releases pheromones that make him more desirable (again going back to the "proven desirable" mentioned above.) This may or may not bode well for the relationship.
- Belldandy for Keiichi in Ah! My Goddess. Keiichi's pseudo-rival is only vaguely interested in Bell because she isn't. Sayoko is arguably less interested in Keiichi than annoyed at Belldandy stealing away most of the attention from her classmates, as well as the protective nature of Bell and Kei's wish causing problems for anyone intruding on them.
- In Ai Yori Aoshi, Kaoru and his childhood friend Aoi secretly get engaged and move into a mansion with the cover story that she's his landlady. Soon a large number of girls move in as Aoi's tenants, and by the end of the story all of them have fallen for Kaoru as well. Both Tina and Mayu loved him already, but the presence of competition motivates them to try harder for his attention.
- Touma in A Certain Magical Index is not an example. It's stated that his teacher and all of his classmates, and a great many other females, are attracted to him, or at least find him cute because of his bad luck, before Index comes along.
- Once Keitaro meets Naru in Love Hina, other women start showing up like it's a haremette convention.
- Mahou Sensei Negima! has an... interesting take on this; when Negi asks to speak to Ku Fei after class, a bunch of the other girls automatically assume he likes her and begin making their own attempts to win him. Of course, after they all work themselves up over it, it turns out that he wanted learn martial arts from her.
- It also happens in a more traditional sense. Nodoka confesses to him and explains to the other girls that she likes his maturity and determination. Cue numerous other girls beginning to be attracted to him for the same reasons. Up to and including a robot.
- Midori Days takes this to extremes - Midori is not only both figuratively and literally attached to Seiji, but her appearance on his right arm sparks a run of luck where quite a few women (and one slightly nerdy boy) fall for Seiji. Justified as Seiji was a Jerkass before Midori started forcing him to change his ways, so his Character Development made him more attractive to the people around him.
- Nagasarete Airantou almost fits this trope to a T. The magnetism, however, is due not to the guy getting a girl, but to the fact that the guy is the guy — the only human male on the island. Hilarity Ensues.
- Himari of Omamori Himari. All the Cute Monster Girls (and one Dark Magical Girl) seem to have come out of the woods right after Himari entered Yuuto's life, although that may have more to do with the awakening of his demon-hunting powers when he turned 16.
- Rosario + Vampire: Nice Guy Tsukune gets a girlfriend on his first day at Yokai Academy, the incredibly gorgeous and friendly vampire girl Moka Akashiya. Its not long before several more girls fall for him: A big-breasted succubus, a young witch girl who also has a crush on Moka, a yuki-onna who's a Stalker with a Crush, and another, older witch (though she's not too crazy over him like the others are).
- Discussed in Spice and Wolf. A barmaid tells Lawrence that having a beautiful "companion" (who even went to the effort of marking him with her scent) makes him more desirable.
- Tenchi Muyo!. Poor naive Tenchi frees Ryoko from her prison, where she has been locked for 700 years. Within a matter of days five or six more show up, almost all of whom are capable of blowing up his house at minimum, the planet at maximum. All of whom are heavily connected to Ryoko, even if some of them don't realize it.
- Lala, Rito's Magical Alien Fiance (of sorts), from the To Love-Ru manga. Before he met her, he was pretty much just some average and nice guy who can't admit his crush on an old friend (who apparently also has a crush on him that he isn't aware of). After meeting Lala, however, a series of events is set off that sees him unwittingly gathering an Unwanted Harem so big that Lala agrees with (her sister) Momo's idea to make an official harem for him!.
- A running gag in the early Video Girl Ai manga and anime is that Youta Moteuchi's name, if you mispronounce the kanji as "Motenai Yo Da," means "Dateless." He did everything in his power to be noticeable, even buying magazines for tips. Once he met and lost Ai, however, everyone from his Unlucky Childhood Friend to the Girl Next Door to a former classmate who had a crush on him (as well as an amnesiac Ai) falls head over heels for him.
- A mild version happens in Seinen manga and Anime Yume De Aetara (aka If I See You In My Dreams). The wimpy Masuo Fuguno falls in love with the beautiful Nagisa, and suddenly a lot of women began to insinuate/try to seduce/trip on him scantly dressed/fall in love with him. Masuo, being a man with no romance experience, doesn't know how to cope with this or convince Nagisa that everything is a misunderstanding; and Nagisa, being almost as inexperienced as him and a bit of Tsundere, doesn't believe him and gives the cold shoulder. Notorious because, even with this setup, it doesn't become a harem series.
- Advice and Trust
- Girls suddenly find Shinji a lot more attractive once he and Asuka open up to each other. Of course, since they're trying to keep their relationship a secret, Asuka can't do anything but glare menacingly and curse under her breath while other girls try to steal her man.
- Rei actually invokes this by pretending to be going out with Kensuke in order to get him a real girlfriend.
- When the usually-isolated Utahraptor packs of Raptor Red are brought together by a field of carrion-scented flowers, Raptor Red's consort, being a consort, attracts the attention of other raptor females, forcing Raptor Red to fight for him. The consort himself isn't exactly thrilled by this.
- This happens to JD in Scrubs. In the course of an episode he goes from dateless, to having a girlfriend and back, while a random woman in a bar follows this to a tee.
- Happens to Mike in Off Centre. It's actually lampshaded and later subverted when he then tries to cheat on his girlfriend. The girls that were all over him suddenly hate his guts.
- This happens to Dean in the season 6 premiere of Supernatural: he's wearing a wedding ring (although he's not married, he is shacking up with someone at the time) and a waitress at a bar gives him her phone number along with the check. Dean lampshades it to his buddy that he's in the bar with, but it turns out it's really a demon trying to get him alone so she can kill him.
- 30 Rock based a whole episode on this, and called the phenomenon "when it rains, it pours."
- In the webcomic YU+ME: dream , once everyone knows Fiona is a lesbian, they kiss her or threaten her to stay away from "their girl".
- Kevin makes this observation in Wapsi Square here.
- Nancy in Ow, my sanity, for unusual (but perfectly logical) reasons.
- Tai of Questionable Content has an interesting theory on the phenomenon.
- A platonic example is seen in this comic of Living With Hipstergirl And Gamergirl. An acquaintance of Artur's, Megan, is talking to a co-worker of his, dismissing Artur before both women notice Artur being greeted by Erika and Sophie. Needless to say, the girl becomes quite interested now. According to a few comments, this is actually fairly common to be a joke in Latin America. While Artur isn't in a relationship with either, they are pretty close and there has been a healthy amount of Ship Tease.
- The episode "Terry's Friend Dates a Robot" of Batman Beyond has a nerd who (successfully) attempts to emulate this effect by purchasing an illegal android girlfriend. He's instantly considered more popular, and girls who already have boyfriends offer to "double date" with him. Unfortunately, the robot's a Clingy Jealous Girl...
- The song "Temptation" from The Last Five Years:
"Everyone tells you that the minute you get married, every other woman in the in the world suddenly finds you attractive; well, that's not true. It only affects the kind of women you always wanted to sleep with, but who wouldn't give you the time of day before, and now they're banging down your door and falling to their knees, or at least that's what it feels like, because you can not touch them..."