A man takes a girl out in his car, stops seven miles out of town, and says he wants to screw her. She refuses and walks back. The second night, he takes her twelve miles out of town and tells her he really wants to screw her. She refuses and walks back. The third night, he takes her thirty miles away, and lo and behold, she gives in.
Afterwards he asks why she finally gave in.
She shrugged, "I'll walk seven miles, or even twelve miles, to save a friend of mine from a case of AIDS. But not thirty, no way."