Because of who you are, I won’t sleep with anyone else. If this involves sacrifice, so be it. Part of what I offer you is the promise that, to the extent I desire others, or other than what you have to offer, I will go without. (N.b., in many relationships, there is an added promise: I will pretend that I don’t desire others, or anything other than what you have to offer.) And you offer me the same.
Because of who you are, I won’t sleep with anyone you’d prefer I not sleep with (usually because you’re uncomfortable with the closeness between what she offers me and what you offer me, but sometimes because of other things going on in your life, my life, or our collective life, at any given moment).
And, I understand and accept that there may be people who turn you on in ways that I don’t turn you on. And that’s ok with me.
And you understand and accept that there may be people who turn me on in ways you don’t turn me on. And that’s ok with you.
Surely, some monogamous relationships incorporate the two paragraphs just above this one. How many? What proportion? I have no idea.