People get upset by 'white knights' because it shows them how much they actually let slide. People's mental image of themselves as 'good people' is important to them. But every sexist comment that they let be posted without raising an issue detracts from that self-image.
It's a kind of a lie people tell themselves to keep that self-image intact. The idea that the person is only helping someone to sleep with them makes them an inferior person, and thus we're are better than them, even though we've done nothing.
Of course, the argument against 'white knighting' is that women can look after themselves, and don't need protection, and it's inherently sexist to try and help them. Well, if you saw a woman being attacked on the street, would you just keep on walking? If you saw anyone being attacked, would you just ignore it? Well, maybe. But that's more revealing about you rather than the person running to help. That argument is just kind of silly.
In the end, 'white knight' hate is fairly irrational. You get angry at someone unrelated to you for helping a stranger out. People are going to try and twist the definition of the term to justify their anger... but rationalizing inaction and bad behavior is basically part of being human.
And if you still want to argue about peoples motives: People are judged by their actions.
