Fire Emblem fandom is stuffed with this, especially the 4th and 7th installments. Then the 13th came in and blew them outta the water.
Word of advice to forum newbies? Do not, under any circumstances, bring up the subject of Roy and Lilina's mothers. Or Wolt's father. Heck, don't question or mention the parentage of any kid whose name isn't Klein, Clarine, Hugh, Igrene, Rei/Ray/Lleu/Raigh, Lugh, or Fir. You'll be sucked into a neverending stream of arguments and namecalling.
The FE6 discussion is insane. Rebecca was Wolt's canon mother, but the father had to be either Wil (Guy Next Door, Nice Guy), Lowen (fellow cook, Nice Guy too, saved Rebecca from bandits so she adores him ever since then) or Sain (Chivalrous Pervert) — and the kid resembled the three of themnote (He has Wil's Archer class and hairstyle, Lowen's personality and hair color, and Sain's eyes/features) Lyn is the prime core of the battles, due to her 4 romantic options (or 5, if you're a Les Yay fan for Lyn/Florina): Kent (Bodyguard Crush), Eliwood (The protagonist), Hector (the bickering they have every other chapter in Hector Mode, along with their conversations in 29x/31x and the Hector's story endgame if they have an A support, in addition to Lilina having both Lyn's frontal bangs and the name "Lyn" in her name albeit with a different spelling), and Rath (due to the existence of Sue in FE6). Hector is also madness, with 3 wives: Florina (Bi the WayShrinking Violet, really wants to thank him for saving her and might have a crush on him), Lyn (explained earlier), and Farina (Florina's middle sister, exclusive to Hector Mode, recruitable in chapter 25). Eliwood has 3 wives as well: Ninian (according to the Marcus/Lilina supports in FE6, a few slight hints in FE7's main story, and him saving her at one point in Lyn's story), Lyn (STILL explained earlier), and Fiora (several lords of Fire Emblem can be paired with Pegasus Knights or have Ship Tease with them, possible canon from the Marcus/Lilina supports in FE6 since they involve her homeland Ilia).
Plus, even Rei and Lugh could be argued, since their mother Nino has two potential husbands. There's enough in-game evidence to support Jaffar as Nino's canon love interest, but she can marry Erk if you get them to A support. And their supports are just as meaningful and sweet as the Nino/Jaffar ones, even without outside interaction. Oddly enough, though, this doesn't create as big of an argument as discussing the Lords' pairings does.
That might be because the wank fuel goes to the Erk/Priscilla vs. Erk/Serra ship wars instead. Priscilla? Stupid, bland, idiotic Purity Sue! Serra? Bitchy, mean, shallow Jerk Ass! You could go on for YEARS in regards to that one.
Actually, Jaffar and Nino is often bashed and falsely accused of being lolicon and creepy. Thing is: Jaffar's age has NEVER been brought up in-game, yet he looks to not be past early 20's... and Nino is 14, not 11 or 12. Pedophilia it ain't, people. Not to mention that anyone who bothers to do the math should know that Nino was 20 when she gave birth to the twins. Unsurprisingly, the major part of the Jaffar/Nino bashing comes from Erk/Nino rabidshippers who think 15-year-old Erk is inherently better than the older Jaffar due to his and Nino's similar ages, ignoring that Erk has his own issues as well.
Given that Genealogy of Holy War made shipping a game mechanic, we probably shouldn't be all that surprised. Never ever mention names like "Lewyn", "Raqesis", "Ira", "Sylvia" or "Tailto" unless you want to be thrown into a burning fire about who should go with who, the advantages of pairing one and another, etc.
The Ephraim/Lyon fans tend to be quiet yet friendly in general. However, some of them seem to be very jumpy whenever they feel threatened. In particular, some of them seem to either deny or minimise Lyon's feelings for Eirika and Ephaim's Ship Tease with Tana, claiming that "Ephraim and Lyon are much more tender with each other than with women so they're interently better".
There have been more than a few terse words over who Ike might possibly end up with, though it tends to be divided between people who ship Ike/Soren, and those who deny that Ike could be anything other than 100% heterosexual. It's usually not TOO bad, unless someone happens to mention that Ike shows zero romantic/sexual interest in women and that Ike/Soren is very strongly hinted canon. Then things can get ugly. Oddly enough Ike/Ranulf gets a pass, probably because it's not nearly as popular and neither side has as much "evidence" for their case.
As of now, the biggest fight leaves Ike/Soren aside and get behind Ike/Elincia and Elincia/Geoffrey shippers. Some Ike/Elincia shippers are simply brutal to Geoffrey, and one of them brought up a massive wank when she wrote a fic in which Geoffrey burned museums and killed innocent people because Ike and Elincia were together. Not helped by her using mental illness as an "explanation" of Geoffrey's "issues".
Awakening marks the return of the Lover/Inheritance System from Genealogy... and the cannons immediately started firing. Tip: Don't mention "Nowi", "Tharja", "Olivia", "Cordelia" or "Sumia" in regards to pairings- unless you want to have your brain melted... violently. (Furthermore, don't mention Sumia's daughter Cynthia, The Avatar's child Morgan or Chrom's daughter Lucina in any given situation, or you'll cause an Internet Backdraft.) The rabidness of some Chrom/Female Avatar fans does NOT help.
Sumia gets massive bashing for being the most ship teased love interest for Chrom aside of Female!Avatar. It's often full of misogynistic double standards, like: calling her a "slut" or a "whore" for being adorkable around him and having one of the best shots at being his Love Interest; or falsely accusing her being a Satellite Love Interest who "only exists to obsess over Chrom and throw herself at his feet" and "brings back feminism xxxx years", for either "trying to buy his affection with pies" or "stealing the man that Female Avatar DESERVES more as her lover for being a strong woman". It's not helped by how some Gaius/Sumia and Frederick/Sumia fans pour passive-aggressive hate on Chrom/Sumia, while portraying their guys of choice as either a dashing Lovable Rogue or a Knight in Shining Armor who will flawlessly sweep poor little ditzy Sumia off her feet and "heal" her from her impossible love. Some of these fans also happento be rabid Chrom/Female!Avatar fans, so the intentions behind this are obvious. (Chrom/Sully fans have also been accused of this, but so far it hasn't been spotted.)
On the other other hand, the Female Avatar is not free of this either. Many fans have lashed out at the popularity of Chrom/Female!Avatar and both the disturbing levels of self-projectionand the utter bitchiness towards other characters/pairings (particularly Chrom/Sumia and Sumia as a character, and sometimes against Chrom/Olivia as well as Cordelia's Unrequited Love) that the most rabid fans display. They do it by accusing the pairing of being Strangled by the Red String as well as ship teasedeven after Chrom marries someone else. (i.e., Lucina practically accuses a non-Chrom-married!Avatar of being Chrom's "other woman" and is shell shocked when she isn't.) Or by joining the fandom "faction" that insists on calling Female!Avatar a Canon Sue and referring to all of her fans as creepy and overinvested whether they are or not. (Hint: they aren't) Or by endlessly complaining about how the loudest fans insist they're "deeper", "more meaningful" and "more dramatic" than all their other pairings.
Some Henry/Olivia fans tend to get very defensive and bitchy against Henry's other prospect love interests and claim Olivia is the only one who truly loves and accepts him, completely handwaving possible problems coming from Henry's weird view of the world and Olivia's lack of social skills united to her shyness. Any reminders that Henry/Olivia isn't fixed canon and/or that she isn't the only love interest he's mentioned his Dark and Troubled Past to will be met by these rabidshippers with accusations of statwhoring, catty "you're just jealous of our ship's popularity" remarks, etc.
The Lon'qu/Lissa fans are rapidly starting to fit in here, specially due to their rabid insistence that it's the intended canon pairing for them despite how either can hook up with others if the player so desires. Some non-fans have even been bullied for merely saying they don't ship it.
The view of Cordelia in fandom can be easily integrated into this. Some fans accuse her of being a Chrom-obsessed slut who would jump at the chance of being Chrom's "other woman" and, if married to another guy, cheat on her husband with him. (Again, the most rabid Chrom/Female Avatar seem to be the worst about it.) Or, in the view of some Chrom/Sumia fans, capable of breaking off her years long friendship with Sumia out of spite over not being the one favored by Chrom.
In regards to the last phrase, some parts of the fandom seems to be hellbent over creating a fanonLove Triangle between Sumia and Cordelia over Chrom: either Sumia is a heartless and selfish bitch who totally steps over poor victim Cordelia's feelings and "steals" Chrom from her "stronger and thus more deserving" best friend, or Cordelia is a Chrom-obsessed slut who (as said above) would cruelly dump her sweet and devoted best friend if she ever dared attract Chrom's attention. And this despite the game going outta its way to avert the "Love Triangle" trope and remarking on how Cordelia and Sumia's friendship is pretty much unbreakable.
Even worse, do NOT try to decipher what Priam said about being "a descendant of Ike". The wording is vague already, but MANY fangirls started screaming in horror since they assumed it HAD to mean that Ike/Soren was automatically jossed, while smug anti-shippers clapped their hands in glee for the same reasons... while the brainier members of the shipping faction pointed out that the series has already misused the term "descendant" before (regarding the Heroic Lineage of Falchion actually being descended from Anri's brother Marcellus since Anri himself was a Celibate Herofor very messy reasons), thereby setting a precedent that would allow for Priam to actually be descended from Ike's little sister Mist and whichever of her potential love interests you prefer and Ike/Soren to still be viable. On the other hand, Priam uses Ragnell, Ike's sword, and as Ike took Ragnell with him when he left Mist behind in order to go on adventures. So the whole issue is forever open to debate, and these debates can get messy.
In the Ho Yay fields, Gerome/Inigo is both the most popular and the most disliked same-sex pairing in the game. The "reasons" are: the apparent misogyny of the fans since the louder ones let Inigo's flaws slide while bashing his prospect female love interests for "being mean to him", the fetishization/glamourisation of same sex relationships that leads to many fanworks slapping OOC uke/seme dynamics on the two regardless of their actual in-story personalities and motivations, and/or the hissy-scratchy reactions of some fangirls at the mere thought of Inigo being paired with anyone but Gerome, be male or female.
As a side note, to be fair to the people bashing Inigo's love interests for "being mean to him", at the very least they definitely have a point with regards to Nah, who is incredibly(and uncharacteristically) abusive to Inigo, and even forces him to marry her under threat of eating him alive. Even more jarring because Inigo doesn't seem to like her as more than a friend, and because while Nah can act clingy and immature with some of her other love interests, she never treats the other boys that badly.