Feeling jealous of partner’s close friendships with opposite sex?

My partner and I are taking things to the next level in our relationship. We plan to get married, and I trust that he is committed to me. There is one area that I can’t seem to let go of though, and that is his close friendships with women. Before me, he didn’t have a relationship, but cultivated family-like friendships with several women. He has explained it was always just like a family friendship, but I feel threatened over some of the connections. I’ve never met them, but he did tell them about me early on, and since we started dating, he’s minimized his contact. Seeing pictures of them all together can trigger jealousy sometimes, and when there is occasional contact, like around the holidays. They are in their own relationships, and always had been, so I trust there was nothing more than a friendship, but it’s a bond I feel threatened by because at one time, they were all very bonded and comfortable, maybe even like siblings. I’m not sure if I’ll ever be able to let go of this feeling.