I think the fact that you are unsure that this boundary would even make you feel better makes it a bit arbitrary and not reasonable. You should be cautious about asking your partner to make major alterations in his style of sex and intimacy.
You are basically asking him to behave in a way that is unnatural to him. Certainly it is impossible to avoid doing this sometimes, being civilized often requires us to act in ways that go against our raw impulses.
What is it about casual sex that bothers you? Is it possible that you are simply stalling, hoping that the sex will never happen at all?
You aren't totally comfortable with poly, and that's perfectly understandable. I don't think these boundaries will help with that. You need to figure it out a bit more, IMO, before looking to apply cures.
Hinge of a V relationship with my husband (Thumper) and boyfriend (T-Rex). Also, mother of a 6 y/o girl by my husband.
My poly story begins here
. Now with new blogging