Honestly, if you aren’t comfortable with your SO fucking another girl, you shouldn’t settle for a partner who would even consider it. You should find a normal person who is satisfied with one sexual partner. The kissing and cuddling that ensued after they had sex, the texting, and his desire to fulfill this fantasy demonstrates he views her as more than just a friend. I would bet a lot of money that if you all broke up right now, he would try to pursue a relationship with this woman. He was considerate (I suppose?) to ask more than once before the sex, but he doesn’t sound like the right boyfriend for you. He’s only making you more insecure. You aren’t comfortable with your boyfriend having sex with another girl, and that’s okay and normal. He wanted to fuck another girl, and to me, that’s demonstrates you two have a major compatibility issue. I think it’s time to split up.