Would you let your bf remain friends with a girl he hooked up with (kiss,touch)

My bf and I been together 3 years , he has two female friends he has known since high school. The one I'm OK with them being friends but the other he hooked up with while him and his ex were on a break. In the beginning he went behind my back texting her , but says they are only friends , and he misses there friendship. He does not live it down that I won't let them accociate. It's come to a point now that he knows I'm uncomfortable but wants to prove me wrong that it's not like that , so he can have his friend back. He is willing to make me be uncomfortable just to see this girl again and is using that fact that he has no friends as an excuse so I feel bad. I finally gave in and told him if it's that important to him do it just know it at put a strain on our relationship. Well he did reach out to her and is trying to hang out . Am I wrong for saying OK. It's come to a point that it was no longer an option and I got the feeling he might do it behind back if I said no still. Any advice ??