Boyfriend and his Girl BF

My boyfriend and his current girl best friend have been close for 4 years. Me and him have been dating roughly 2. Tonight, I found out that when he sleeps over, they share a bed. I don’t think there’s really anything going on because she is the on-again, off-again gf of his cousin, but it still bothers me. He mostly stays in my home town with me since I’m home from school because we’re usually long-distance

so I understand him wanting to spend extra time with his friends when he’s in his hometown. Also, he lives with a distant relative and they’re hostile to him. The thing is when I told him I didn’t like it, he was like, “Well, it’s been that way since high school.” I’m lost. All I could do was sit in silence. Help.