Bestie With His Ex GF

My boyfriend is friends with his ex girlfriend. They've also had sex when they were dating. It was one time though from what I was told. My boyfriend told me the only reason they dated was because they were really close friends and so were their parents and their parents kept pushing them to date. They finally decided to give it a try. When I asked why they broke up, he said it was after they had sex. They decided since they had been going on out they should try sex. He said how it went was, imagine if you got shit faced drunk, and woke up in bed with your brother. Well I didn't need THAT image in my head! But I was trying to understand what he was saying. He said they broke up that morning and didn't talk for like six months. He told me that it almost broke their friendship. They did reconcile and start being friends again. All this happened 5 years ago. I'm glad I have a bit of back story, but I still don't know how to feel about it.