It's quite arrogant to say that just because you leap to a womans defense you're a bad person? Probably been mentioned, but I'd leap to the aid of a woman in distress any day, whether she's being mugged, raped or beaten. Same thing applies on the internet to a certain degree.
Regularly, not always though, men have more power on the internet and has a better chance of making a difference.
A man who's being abusive towards a woman isn't going to stop because the woman tells him to stop, she's in essence defenseless against the disgusting behaviour of sexist pigs on the internet, with the exception of just ignoring the guy.
And I have no idea of where you're getting your facts, but these "nice guys" that you mention, which I am a part of, don't just leap to the aid of women, but people of either gender and any age.
Why? There's power in numbers. I was bullied in school for years, think anyone ever helped me or even cared? Nope. Would I like someone to care? Yes.
So I'm the kind of guy who immediately reacts to someone else being abusive to another person, because it's immoral and stupid.
Even if you can't get him to stop, you can get the abused person to know that he or she is not alone, and that there are good, decent people around.
And yes, I've become attracted to women I've defended in such situations, but generally I don't. I am in fact a nice guy, so I will be nice regardless of gender and possibility of sexual conquest.
So what you're doing is generalizing a large portion of mankind based on what I would believe is your own perception of your own personality.
I'm fairly certain that you're either of two possible choices. An abusive sexist man who hates guys who defend women, or a guy who defends women but never gets laid because of it.
Perhaps I sound cruel when I say I believe that, but why else would anyone in their right mind believe that all men who aid others are sexist ponces?
It makes no sense.
Should we start calling administrators on forums and gameservers sexist geezes because they punish the wicked? Where does it end? At what point does it become okay to care for other human beings?