As has been said a number of times, part of the point of "fanon" is you can run with it...if you want it, if you can imagine it...BAM! It's there. Personally, I feel like the concept of Cloud having a romantic attraction to Sephiroth undermines if not outright ruins a lot of the game's plotting and drive, and several of the central themes involving the nature of revenge/childhood idolization versus adult disillusionment, and, finally--hokey as it may sound--the need by all humans to love and be loved, and the attainment of that as part of what makes a person whole (Sephiroth, eventually even his 'idolization' version representing hatred, and Tifa representing love). I suppose you could argue that that means this concept just clashes with MY fanon, but...I can't help but feel like it's clashing with the actual script, rather than my brain, lol. Am I just being too prideful here?
Unless, again, the suggestion is more simply as to whether Cloud is potentially bisexual in general...which besides there being no evidence for or against, is abstract and irrelevant enough so as to be entirely pointless. We may as well be contemplating whether he likes crunchy or smooth peanut butter.
(Fucking SMOOTH, baby)