I need help with how to handle this situation

My boyfriend and I have been dating for over a year and there’s always an issue that seems to come up. There’s this girl that he became friends with through a class that is VERY flirty with him I thought nothing of it until I saw that he was flirty back with her, innocent flirty but still flirty. They talked everyday and that’s when it started to bother me knowing that they were talking all the time flirting back and forth with each other. He was also sharing information about our personal relationship including our sex life with this girl. I confronted him about it and he says there just friends and that he liked talking to her because they got along well. We go into a big fight and he agreed he was wrong for flirting with her and told her if she didn’t stop flirting with him that they would have to stop talking. But ever since they had this emotional relationship I dont want him texting her at all because their relationship really hurt my feelings. She still texts him occasionally and he says that she is his friend and that he can text her if he wants, which I know is true. I need help with how to handle this situation, I’m 100% ok with him texting and being friends with other girls I just don’t know why he wants to talk and be friends with this girl who was very disrespectful to our relationship by flirting with him so much knowing he had a girlfriend.