Bi-polar

Hey guys me again. I’m having a rough patch in my relationship. So I saw a Snapchat of my boyfriend calling his female friend of years “sexy”. He’s very loyal to me (from what Ik at least) and hasn’t done anything to jeopardize our relationship. We go into a whole argument and He swears to me she’s just a friend. I believed him somewhat so I decided to follow her on a social site. She followed me back and liked a lot of my pictures. She even messaged me and told me she wants to meet me. Now he’s mad she wants to be friends with me and I feel like an ass. But my point is why is he mad if I apologized and just trying to keep the peace?? Pls help