Boyfriend wants to have a threesome

Okay so a couple months ago my boyfriend found out that I’ve always wanted to experience being with a woman. He then suggested we have a threesome. At first I was completely for it because he said he only wanted to watch me have sex with the girl and he would ONLY have sex with me. I’m currently 21 weeks pregnant and I told him that I would rather wait until after the baby is born to do this. Well now he’s saying that since I want to wait he also wants to have sex with the girl because “that’s how threesomes work”. I’ve told him multiple times why I feel insecure about doing it now and him having sex with both of us. He then brings up how he doesn’t understand the difference between me having sex with two guys and him having sex with me and another girl. Mind you I had a threesome with two guys a LONG TIME AGO during an awful time in my life and I’m not proud of what I did. That happened years before I met him. I don’t know how to handle this anymore and I honestly think maybe we’re both too insecure for a threesome. I don’t want to fight with him over something so stupid.

UPDATE:

We both decided we were too insecure to handle a threesome. He also expressed that he didn’t want me to leave him for a woman (which I would never do) and that’s why he acted that way. We have worked it out and figured out that we need to spend more quality time together. Thank you all so much for the comments and support❤️