Just a week ago I had that situation with a girl who seriously should have been a solid one-night stand. But she got hers and I was left dry. Good thing it was early morning, I was still fairly pissed and tired so I just went home to sleep...otherwise there could have been a rape attempt reported next day.
This is precisely "I'd sleep with a hooker but I wouldn't marry her" mindset and before you wonder, for those people, girls who have sex before marriage are prostitutes.
I don't know the reason but I know irreligious people who think like that.