. . . I could draw the lines, watch her leave again and lose her. . .
I can see you don't understand boundaries, BG90, or "N.U.T.S."
To me, that's the beauty of boundaries, or -- as I have heard someone else call them -- "My Boundaries of Personal Integrity." Only YOU know what they are, but they should be a very short list; your "dealbreakers," as it were . . . those things that you, as a person with your values, morals and ethics, simply CANNOT ABIDE.
And this is how it works, in practical application: If you decide that "I will not live in an open marriage," and you state that as a boundary to a cheating spouse, and if that drives them away from you, and toward the other person? Well, then that's THEIR CHOICE, and them cheating -- and staying with me -- wasn't an option for me anyway, based on my own authenticity and values, so what have I lost?
All I've lost in that instance is something that I could not have abided anyway.
"You must choose between her or me" is an ULTIMATUM. It's about THEM.
Boundaries should be about YOU -- "I will not live in an open marriage." It's then up to the other person to decide what to DO with that information you've now conveyed to them so earnestly.