It might be only a small comment or joke - but all of them together form a massive pressure on women. For the one making the sexist comment it's always "a little joke", where in fact all those little comments build a world where women have less opportunities. No female student should have to stand against the professor. You are there for studying.
But regardless if you study or go to school or go to work - there is a constant flow of little sexist comments trying to keep you away from what you are supposed to do.
I really wonder about the intelligence of these people who need to build their self esteem from that.