I'd say that most of these 'layabout' girls were probably just sleeping with you to be perfunctory. They probably weren't all that into you - I know that when I was younger, there were a couple of times where I would just 'do it' to get it out of the way if I was bored, slightly horny & felt the conversation was going nowhere. Now, I have no qualms in rejecting advances that I'm not 200% keen on - I have to be really into someone, for my own sake, to know that I'll put in maximum effort. That said, I've had an even bigger number of encounters with strangers that were totally red hot, but those came later - when I took more control in exercising discretion (by taking a more active pursuit in playing the field).