My fiance ‘25M’ agreed to give his ex girlfriend another baby after I ‘23F’ said absolutely not. How can I forgive him for something so insane?

My fiance (25 male) has an ex girlfriend that he has a 4 year old child with. His ex girlfriend is now a lesbian and engaged to a women, living in a city 7 hours away together with that child. They called us over FaceTime 2 weeks ago and asked us the ridiculous question if we would be comfortable if my fiance (her ex boyfriend) would be comfortable donating his sperm to them so his ex and her new female fiance could have a baby together and it would be related to the child. I’m obviously so uncomfortable with that an couldn’t believe they even asked that question an we respectfully declined. I told my fiance all the reasons I was uncomfortable with it and we agreed that it wasn’t going to happen. Fast forward to the past few days. My fiancé and I have been fighting a lot and have decided to take a break. While we were on the break. One day after I left he called his ex girlfriend and told her he was willing to donate his sperm so they could have a baby behind my back. He calls me the next day after their conversation (today) while we’re still on our break and tells me he said he told her he’s willing. I freaked out and told him we are officially over and he’s ruined everything. And he said it was a mistake and he’ll call her and tell her he’s changed his mind and he’s not going to do it cause he doesn’t want to lose me over this. I just feel like the damage is done and I don’t know how to feel anymore. How can I forgive him for something so insane?