To me, a mono/poly relationship is already a compromise. It's not "the poly person can have outside partners so they win" vs "nobody has outside partners so the mono wins". Because to me, the equivalent of not allowing the poly partner to have other relationships is forcing the mono partner to.
When the poly partner is allowed outside partners, and the mono partner is allowed not to have any, then both parts are compromising, the way I see it.
Do I want to date someone who is mono? Not if I can choose! I wouldn't want to be their only partner, be pressured to be their everything, be resented for loving more than just them, and not have metamours to rely on. Nor being able to share some common poly things, such as talking about the people we are interested in or discussing poly news/networks, etc.
If a partner insists that I don't have anyone other than them, it's not a decision I made not to add partners, as could be the case if I was polysaturated or wanted to care for children or a relative or something. It would be something they decide for me. It would be them controlling who I'm allowed to love.
And as heartbreaking as it would be, I couldn't stay in a relationship with someone like that. Because they wouldn't let me be myself.
Right now, I have a poly partner, and I haven't had anyone else since separating from my husband two years ago. But I'm poly, and that means I'm allowed to, and that freedom is worth everything. It makes the difference between a happy and an unhappy relationship. It makes the difference between feeling miserable for not being allowed to ask someone out, and feeling free and fulfilled because I have that option - even if I decide not to take advantage of it.
It would hurt me because they would want to control not just their end of the relationship, which is their right, but mine too, and they would feel entitled to it. And that's just a completely different state of mind than mine. It's fine if you have control over an aspect of someone else's life that they want you to have control over, as in mutual monogamy. It's not if it's against your will.
I would only ever be able to date a mono person who is fine with my being poly, for whatever reason. Of course, because they're mono, I'd be scared that they'd just leave me when someone else comes along, but I guess that's a risk I'd be willing to take if I really liked them.