First, it is rather rude to dismiss entire sections of fanfiction, some of which have been very well written, merely because you see Ranma as a guy and don't think he could ever fall for another guy. It is especially rude to be quite so offensive in how you posted it. Posting that you do not understand these fics and would like someone to defend them would have been perfectly reasonable, and is the imaginary post I am going to be responding to.
For Ranma falling for another guy, several things need to be considered:
One is that quite a few fanfictions are deliberately made AU(alternate universe) where Ranma is a different person in either small or large ways.
Second is that Ranma's sexuality is never explored in the series, except to show that he is rather uncomfortable(often for good reason) around people of both genders when they are pursuing him romantically. He has a very overblown concept of masculinity and exactly what being a guy does/should entail thanks to his dad and the way he was raised. He is also very insecure about his masculinity, as his father has attacked him constantly by belittling him as being 'like a girl' for most of his life, and suddenly he spends half of his time as one. Together these make Ranma admitting to attraction for anyone very unlikely, with a guy being less likely than a girl, but that does not affect who he is attracted to.
Third is the very real possibility that the nyannichuan messed with his sexual orientation, making him either bi or heterosexual while female, and possibly affecting his male orientation as well.
Specific pairings depend on how well written they are. I've seen one(just one) believably write him winding up falling for Kuno Tatewaki as a girl. That particular pairing is very difficult to make believable, though.
In short, the idea does not even stretch credibility, much less break suspension of disbelief, unless poorly written or because you have some personal aversion to the concept. It doesn't even need to violate canon for some of these stories to be completely plausible. I read one fic where how much time he spent in each form determined his orientation: 50/50 he is bi, the more time he spends male, the more he is attracted to girls, and the more time he spends as a girl the more he becomes attracted to guys.