"I will not sleep with someone that is involved with someone else."
You can( but don't have to but should) state this boundary. But the more important part is to live it. Get it in your in your core beliefs and values. You enforce it by your actions.
I really like how LW stated it. It was about you and not them. Much better than "I will not sleep with you while you are with her" type statements.
As a side note. We all have different lines in the sand. When I was single, my line was "Married women are off limits". Probing questions I could find out if the "I have a boy friend" was real or not. Was it a test? Sometimes. Was she living with a guy? If so, then "off limits". Dating is a different topic than we are discussing here, but determining the character of other people is another good skill to have. A found lot of married people acting single during my single days.
"What is best for my kids is best for me" Amor Fati Link to quotes: https://www.divorcebusting.com/forums/ubbthreads.php?ubb=showflat&Number=2879712
