I once was a nice guy.
Nice guy believe that women are pure and innocent, and are afraid of their own desires. Women who hang out with nice guys (and don't sex them) deny those guys' humanity; they deny that they can have sexual desires. When they realize they do indeed have such desires, they might become fag hags.
Nice guys aren't aware of certain facts. Specifically, that most healthy women masturbate. Or, even if they don't masturbate (usually, because they haven't discovered yet how to do it in a way that pleases them), that they have just as weird and naughty fantasies as men do.
Nice guys also don't realize that -- and this is proven by science, namely evolutionary psychology and sociology -- women can display two different personalities, according to who they deal with. They might want to appear as a virginal character most of the time, and turn into the naughtiest slut.
Nice guys also tend to lament on what happens to them without realizing that if it happens to THEM, it's because of THEIR behavior.
Nice guys are usually a byproduct of the politically correct mindset induced by amerifeminists.
See AFC.