The first thing that comes to mind is Love Rain. I think this is almost exactly what you're looking for. However, there's also a lot of melodrama related to their parents, so only 5-10 episodes (I don't remember exactly) are really about the good girl-playboy dynamic.
Some other dramas with playboy MLs, but where the conflict isn't necessarily related to whether he will be faithful.
A Love To Kill
My Secret Romance - Of these 4, this is probably the closest to what you want.
Last Cinderella
The Innocent Man - Not exactly a playboy, more of a gigolo.