Of course she is always in the picture - if she wasn't she and your boyfriend would be lousy parents. People who have children together will need to communicate with each other for the rest of that child's life - not just for the first eighteen years but after that for marriages, grandchildren, great-grandchildren, etc. You are going to have to deal with it. If you want to have a good relationship with your boyfriend, you are not only going to have to deal with it but are going to have to encourage him to be a good parent and maybe even become involved in his child's life as well.
Now, is your boyfriend still attracted to his ex? Are you concerned that he is going to want another heterosexual relationship? Or is it just that your relationship with him does not seem as important as his past relationship?
You are going to have to let this one go. The only way you will feel better is to become involved and a supporting part of his relationship with his child and as a parent cooperating with his ex. No matter how deep your relationship with him is, she will always be a part of his life.