my SO's best friend.

This is long, but I really need opinions. 
My SO has a female best friend. We started dating a few years ago, and I had no idea she existed until she texted him once (a couple months or so into the relationship) and I asked who she was. He told me she was his best friend, like a sister. They hung out a lot (some I didn't know about until I questioned some things) and I always had a feeling something had gone on between them. I found out a year later, from screenshots I was sent and confronted him, that they had sex a few times right before we started dating. We had a baby 5 months ago and every time he's home from hanging out with her he will tell me he is only with me because of the baby, and he doesn't want a relationship with me. He says we should just be friends with benefits. He doesn't want a relationship of drama, and the only "drama" we have is when he hangs out with her. I dread it. He says he just wants to have fun. And then a few days of arguing, we are fine again. I just realized a few days ago that it's ONLY when he's come back from hanging out with her. This girl sends him hearts and kiss faces (didn't know this until her text popped up once), just a few weeks ago he sent her a video with the whole purpose of it being sex. He showed it to me also. I didn't know he sent it until he showed me something on his phone and her message showed up as a reply. They hang out a lot at night and talk in the garage. they talk all day everyday, she's told him she doesn't like me. I have screenshots of her saying she hates me because she is in love with my SO. He tells me she would "never" hate anyone. Im so done and fed up at this point. Im so tired of him talking to her every day, all day long. I'm so tired of him coming home and being upset with me for what seems to be no reason. We are going to couples counseling in a few days and I plan on trying to get him to talk about her. I try now and he will not talk to me at all. He gets defensive and upset with me. He says he gets defensive because he "knows I'm upset". I never wanted to be "that girlfriend" that tells her boyfriend he can't have a girl friend, but I'm at that point where it's me or her. Am I able to do that without seeming like a crazy girlfriend? Should I even attempt that or keep trying not to let any of this bother me? 
EDIT: thank you, ladies. I was really hoping he wasn't cheating, but I've been suspicious for a while. I think I'm going to wait until we go to counseling to bring it up (we start monday) and see how it goes there.