Honestly, I think an amicable relationship between the parties benefits EVERYONE.
Perhaps try to figure out why this bothers you so much...

It took several years, but my ex and I are like this now. Fortunately, my DH isn't bothered by it...

If it hurts you that he had a life prior to you, you may want to work that out before pursuing this relationship further. You can't re-write history

Trust me, them being friends will be so much more peaceful for you. They are parenting together, so she will be in your life one way or another. Better on good terms. As long as he keeps their relationship to a parenting one only, I wouldn't worry. If they start hanging out together, that's another story. He needs to respect your relationship.

Like Oi Vey said, this might be something you want to work out before you marry this man. If you can't get past it, you'll find out before and can find somebody without any children. However, you are not going to find somebody that has no past, so if it were me, I'd maybe consult a therapist and find out why it bothers me.

I think what you are feeling about his past is very normal. I would consider my ex to be a friend. Our divorce was not ugly. He didn't want it, but he didn't make my life a living hell because he knew it was HIS FAULT. He is a good guy, just was a crappy husband. And I don't resent him for that. That is the beauty of divorce, he is going to be someone else's terrible husband, and he can just be a good dad to our girls, and a friend to me.

I hate to think about DH and BM having a past together, but just try to remember, whatever they had in the past wasn't enough to make him stay. He is with you now, and you are his present and future and that is what matters.

I thought my husbands amicable relationship with his ex wife was a good thing as well....in the beginning....but when I noticed that he would delete ALL texts from her bc according to him "he didn't want anything from her on his phone" which was his way of trying to reassure me that he was not interested in persuing a relationship with her again....when checking his phone though....they would talk 3-4 times a day-so u have a right to have an issue with it if it is about more than the Kids....bc BELIEVE me!!!! If things move forward for u two-the second she has an issue with something or feels like u r now more important than her....that is when all he'll breaks loose!!! So u need to figure out to what extent their "friendship" is....his ex called him in the middle of the day at work to discuss what she should do with her (2) wedding rings n if she should give her ring to her daughter n what his thoughts were....although they r divorced-he is still her husband bc her bf won't be.....make sense??? It took two years n a very near divorce before he changed his relationship with her bc of all the issues it was causing...just figure out the extent of their relationship n do not expect it to change-not after marriage or even if u have children together...bc the "she is the mother of my Kidd-I have to talk to her!!" will always b the response u receive....

He has custody of a son that isn't even his? WTF?

I agree that if they can be on good terms it is a good thing for all involve, but being on good terms is one thing being friends is something different. Just think of what the word friend means and what a friend is suppose to do for you. I don't think that being friends with someone you use to be intimately involved with is really possible. Especially when you have had children together. I think that you need to be careful about pursuing your relationship further and make sure that he has appropriate boundaries with his ex now. If not it will just bother you more and more and start to come between you. The way you feel is normal... it is hard to think that your man had a life with someone before you. I struggle with that myself. I like what someone said that if that life was so good then he would still be in it. I am going to try to think of things that way from now on. Maybe you can too.