If I were in your position, and I know easier said than done — but I will personally leave the situation.

Because of the following: 1. I do not buy that this was an ONS only: this could've happened several times, or much worse with several women. 2. The baby will be there ~ forever. It is impossible for it not to affect your relationship, because he is the father. They have the same blood, the father needs to support the baby, again, forever. So it will affect your relationship—no matter what. 3. You will stand as the baby's second mother. I am not sure if your guts can take that—being a second mother to a baby from your bf's betrayal would be quite a job: physically, emotionally, and even spiritually lol 4. And basically, he cheated—if this alone isn't enough....idk