Baby Daddy Drama

Question…Why do guys think they can always “come back” to the mother of their child? So there is this young lady I know, her child is about 7 mos old and she and the father are no longer together. In fact the father of the child is in a relationship and lives with his girlfriend who believes they are getting married this year. The situation between my friend and her bd is cool, she is not interested in a romantic relationship with him, but wants some level of friendship established for the child. Well the dad has been making passes at my friend whenever he comes over to get the baby. Sometimes his girlfriend will be in the car waiting for him to get the baby, and he will be telling my friend how much he misses her or trying to hug her and kiss her. The other day he told her he didn’t know if he would ever stop being attracted to her and tried to make another pass. He’s pretty much made it clear that he’s interested in at least a physical relationship with her, even though he lives with his girlfriend. Again my friend is no longer interested in him and shuts down his advances when he makes them, but she also wonders how long it will take for him to stop with the games. Is this guy just a dog who wants to have his cake and eat it too? Is it possible that he’s just so caught up in having a baby that he is having misguided feelings for the mother of his child? Or is there the expectation that just because they conceived a child together, he can just get it whenever he wants?