I think the point GP was trying to make is that this isn't necessarily a case of male professors being nice to women just because they are attracted to them (or assume that they might be attractive) and that there is a possibility of sex. (or something to that effect)
It's still 'sexism' if a professor responds better to female students than male students because he perceives female students as being more receptive and grateful for the help. But not necessarily in a negative "I'm hoping to get laid" sort of way.