Interesting fact that most of you probably know: Aerith was originally to become Sephiroth's sort-of girlfriend. As far as I understand, this was during story development, and Aerith's love (or whathaveyou) was suppose to be unrequited, so I still don't know if even in the development phase that Sephiroth was suppose to an interest in such things.
I once had the thought that he wanted to procreate with Aerith, as my understanding of the story was that Sephiroth thought he was the only Ancient aside for her. Aka they needed to repopulate the species. As intriguing as that storyline would have been, I'm kind of glad they didn't go that route, as that would have been rather disturbing. xD
I think we all can agree that after he lost his marbles (materia? LOL), he doesn't show an inkling of interest in any of that; if he still has any biological urges he's probably either ignores them with his iron will, or he takes care of the matter himself discreetly. I don't think he has the capacity to love, even his 'mother'. He's gone beyond something like love.
As for pre-BBQ, I still can't imagine Sephiroth in many relationships. I could possibly see a fling, to satisfy his carnal needs, or because he found a particular woman amusing, but again, that's a stretch even by fanfiction standards. He certainly isn't the affectionate type, although he's not without any caring at all towards those he knows (I'm thinking of Crisis Core here), but going from friendship and comardy to love interest is kind of a huge leap. I'm not sure Sephiroth, even before he went insane, could have made that leap. He was, after all, one of the most astoundedly fucked up characters in the entire Final Fantasy series...and that says a lot!
In an AU, well, anything's possible, but such a writer would have to really work the story to make it happen. I have seen the odd light love affair between Sephiroth and a girl in a story (and one or two guys!) that wasn't completely batshit, but I stand firm on my belief that he's asexual, through and through.
Also, yeah, the idea of Sephiroth getting his freak on is...Ugh!
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