My ex-husband always had to keep another woman in my face (we were about your ages at the time) -- who presumably were just friends. Many were.

But when he and I broke up and he had a new wife, my X and were still friends -- and he cheated on the new wife with me.

Even if no sex is involved between your bf and his female friend, this is not a good sign, IMHO. You see where you stand with him -- he spends more time with her. Why aren't you invited to the beach house? Is this what you want?