Dealing with boyfriend's new baby...

Hi,  
I needed to talk to someone outside of my situation to get a honest opinion. So I got with my boyfriend back in January. Sometime in February, he told me that there was a girl that he was messing with that was saying she was pregnant by him. Before me, he was messing around with about 10 girls after a break up a little over a year prior. So he pretty much screwed himself....now the baby was born in July. I had a hard time accepting it from the start because I know new babies have too much attached to them.  He tries so hard to make me not worry and I'm not really worried but I absolutely hate this girl due to the fact that she has sent him naked pics and tried to get with him while she was pregnant...also I want to have a child with him...but it's not coming so easy after coming off of birth control. Before anyone says it, I know for a fact he is not cheating on me with her. We live together and I track his every move...but I just can't deal well with this. He even leaves me on the phone if he goes for a visit to ease my mind and I know he loves me by things he has done for me and everything. This is the short version but basically my question is what should I do? Do I just need to leave?