Yes. You are definitely overreacting. Of course the other man will always be in her life. He is the father of her child. Unless he is abusive and dangerous, it would be ideal for the child to have some kind of relationship with him. It’s important to the child’s identity to feel that there are positive things about the man who fathered her. Your negative feelings about him are detrimental to the child’s self-esteem and to your girlfriend’s feelings of security with you. If you really can’t get over this, do everyone a favor and get out of the relationship. But if you truly love this woman, I think you can move on.
I ask you: What is all the fuss about virginity really about? It’s highly likely that any adult woman you date will have had some sexual experience. It doesn’t mean that there isn’t room for her to have a special and important intimate relationship with you. Every partner is different. The most important variable is the intimacy one feels, not the mechanics of sex.
I’m guessing that you are feeling a bit insecure about your own lack of sexual experience. Rather than deal with that directly, you obsess about your fiancee’s sexual past. Please shift your focus and deal with yourself. There is nothing shameful or unusual about not having had a sexual partner. Lots of people don’t. It doesn’t matter. Your first sexual encounter with your girlfriend may be a little awkward but so what. Make it intimate and tender and loving and it will be perfect in its own way. Once you do it once, you won’t have to worry about the first time anymore.
I wish you well.