Strangers aren't the people you grow up with. Assuming they have a shared view of the world, of social norms, is something you do at your own peril.

If you will feel betrayed by a guy not committing to you after sleeping with him, don't sleep with him until he's committed.

If you will feel cheated by a girl who doesn't sleep with you after you pay for something, don't pay for anything until you've slept with her

Simple.