Guys with children

So recently in the last 7 months me and my husband of 8 years split. I started talking to another man and sleeping with him. Rebound, yes maybe... We hit it off like almost anyone does for the first few months with a lot of baggage from our exes. Mine went off the deep end when he found out and his did too. His ex unfortunately was pregnant and had a baby a month prior to us dating. A baby he never knew about until a week before she delivered. That's kind of fishy in my eyes so I did not like her from the get go. As months went on I told him he needed to get a DNA test because it was just so weird and he agreed but she would not. So even more fishy, right? She did some crazy things like contacting my ex and getting together with him ect. So things cooled down a bit and he told me they did the test and were waiting on results. Come to find out that was a lie. So I went off. Children are so important and it's so important to be there for your child. So then it took another month or so to finally get her to let him do it. As bad as this sounds .. the results come back and she is his. Mind you I have no children and want a family of my own. With this crazy girl I don't see how it would ever work. After the results he doesn't act worried or try to do anything from what I knew. Today I'm told after screaming being upset that he hasn't tried seeing her, that she has stopped over with the child randomly. I'm just so unsure how I should feel about the whole situation. It's absolutely crazy what she has done and I don't want any part of it as hateful as it sounds. On top of it all he's a great guy but is there something I'm missing. Is it just me? Me and my ex are on good terms now also. He throws that in my face which I can see why but at the same time wtf are you doing to this child???