Boyfriend spending time with other girls

My boyfriend of seven months is really close friends with a lot of girls and it bothers me. The are about 4 girls that he talks to almost daily and spends one on one time with them often. I understand that they're his friends but I don't like him spending time with them because I feel like they're coming into him. I've been around him while he's with these girls and they act really weird and flirty around him. I've told him that it bothers me the way they act with him, but he assures me that they're just friends. Is it appropriate for him to be spending so much time with these other girls when we're in a serious relationship? Or am I overreacting? Should I just let him spend time with them?