Cultural advise about bf's ex

Okay ladies I was not raised in the US so I don't know if this is a cultural thing. I started dating my boyfriend in a long distance relationship. And after several months I found out that he's still living in the same house with his ex. It's a house with three floors. He and his roommate are on the second floor and his ex is on the third floor. They almost meet every other day bc the girl's mails and packages are delivered to the second floor. I pointed out that this is really weird and I can't accept that. But he said that it's common in the US. What do you think????