I think that we all here agree that giving up a seat for somebody is an act of courtesy, but I dont think a person should get a seat just because of their sex.
If a woman expect a guy to give up a seat, then she should be expected to do chores and stay home(which raises the question of why she is on the bus, but nevertheless..). Then we go back to the "traditional roles" of women.
LINK for an article about why women should be housewives.
This may be far-fetched, but this could be a form of discrimination (based on sex) that you would give up your seat for a woman, and not a man.
I'm not saying that I wouldnt give up the seat for my parents/relatives/friends etc..., but I would not give it to a random person, just because of their sex.