My brother caught me having sex on his couch? how would you respond.

I'm 18, but still in high school. I'm living with my brother because my parents are doing research abroad. I've been here for about a month and we have gotten close. We fought once when a friend dropped me off drunk but other than that we got along very well. I started recently going out with this guy, my brother doesn't know he's my boyfriend, and the other day my brother said he was going for a long bike ride but it started raining, so they came back. Anyways my boyfriend and I were ass naked on the couch basically having sex. He's very protective and I get what I did was wrong. If this was your sister what would you do?
My brother caught me having sex on his couch? how would you respond.
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