So my boyfriend wants to have sex with other people.

At the start of our relationship we were young, kid less, and a little on the loose side. Now we have 2 kids live together talk about marriage, but.. he still wants to have causal sex with other women. He recently was caught planning to meet up with a girl who also has a boyfriend. He says to him it's the challenge of getting the girls to say they'll do it and then he cancels which he did. I saw the messages where he himself says he cant do it.

My question is.. if I knew he wanted this PRIOR to the kids and marriage talk, do I have the right to tell him I can not accept it?