Why, oh why is it that if a man stops to assist a woman in someway, even if she didn't ask for help and didn't need help, they always expect a sexual favor in return?

That has been my experience, any way. I’ve had, probably, 10 flat tires in my life. I know how to change a tire. Invariably a male would stop to “help” me. Even if I said “No thank you. I have this,” they would insist.
Then they would get angry if I didn’t reciprocate by going out with them, or having sex with them.

Why do they do that?