If you meet a really pretty girl, she's extremely nice to you and you've passed the stage of intimacy & sex then it would be odd if this kind of thing didn't sometimes happen. For me I have my "golden rule" - I never see the same girl twice in a row. Of course it gets broken, and I broke it recently with a really nice girl BUT just having the rule makes me more aware of things, and secondly I did try and book 2 other girls in the intervening days and got let down by both, and was disappointed to be let down. That says it's safe enough in itself, and it won't be 3 in a row no matter what.
The other thing is knowing where the line is. I was recently with a lovely girl and she was discussing her other career and then told me where she worked - I just about freaked and mumbled something about outside world and stuff and probably looked a bit weird. Basically you have to keep that line, you have to be totally aware that what you are doing is NOT having a relationship, NOT having a friendship - it's a business transaction an you need to keep the line there.