I feel so broken after he said this.

My boyfriend keeps bringing up wanting to do things me and him both with other people. He has brought this up so many times before and already knows I really would rather not. I don't know what to tell him anymore. He can't just accept it. He says when we get married people always regret not doing this type of stuff. I'm only 16. He's 17. I don't want to do things with other people. Every time he brings this up, I break alittle more.