I don't think all "White Knights" carry out the behavior that they do, for the sake of receiving some sort of sexual favor. Maybe, as mentioned earlier, it's a way of bonding or developing a relationship with a female. But flat out sex (photos, sexuality, etc)? Nah.
I think some of them are just genuinely interested in pleasing females (and possibly males alike), for the sake of coming off as a sweet and all, and in exchange receiving some sort of respect or reciprocity (Not exactly the terminology I'm looking for, but I think it works). I do a favor for you and help you out from the "bad guys" who are picking on you, and maybe you'll do the same for me some day (or respect/like me).
I think white knights are usually young or simply naive, and so that's why they're willing to risk their own status for the sake of another (usually females). They don't know any better. They're young men who want to be liked or respected and go about doing so in a super-generous (and often self-harming) way (thus making them look weak or foolish).
Simply put, I think "white knights" are what you would call the "nice guys". Just because they're being a certain way doesn't mean they're looking to have a blowjob in the end.