True enough. I agree with you. Though, if you allow me to nitpick, I would say that there already are some gay relationship in the Fire Emblem universe (but nothing really explicit). Legault from FE7 seems to be gay in some of his support conversations (even going as far as calling Morph-Lloyd handsome if you pit him againt it). The bandits from the desert chapter of the same game also seems to be gay (even sporting an A support - though, well, they are kinda stereotyped... And Jasmine may be a combat sadomasochist). There is also Ike and Soren (FE9 + FE10), though how canon that pairing is is left to our imagination. Heather (FE10), despite not having that much lines, is pretty much explicitly gay (even more so in the japanese version of the game, if I believe
my source). Lucius and Raven may also end up in a relationship depending on how you interpret their A support ending (though, I consider that they are more heterosexual life partners than anything).
Gay marriage in a video game like Fire Emblem would be an huge thing. I'm not sure we can expect that from Nintendo yet.