. Or do women flirt more when they know the target can't do anything about it???

Well, not exactly, but you're getting at one of the basic principles of male/female relations. When you have a steady situation (be it marriage, engagement, or long-term commitment) you are more confident, less worried about picking up chicks, etc. So you appear more desirable. Women who might not give you a second look suddenly can't resist you. Plus you're older now than you were then, and as we all know, guys get better with age (to a point, obviously.)

I don't know that this is actually a documented fact, but it's pretty well understood that if a group of guys goes out, it's the married guy that's going to get all the play, while the single guys try to feast on the scraps. Best to just accept it and hope that the girls have hot friends (often the case, thankfully!)