Hi This1. Welcome to PC.
LovelaceF brought up a great point...it's generally a bad idea to compromise core beliefs like that, but it is a good thing that you're concerned about fairness for her.

That says a a lot about you. I would cut her a bit of slack with hugging though, but otherwise I follow your rationale.
I do think though you may be looking at incorrectly. I'm not trying to make any sort of judgment call in the matter, but I think your separation of logic and emotion regarding multiple partners isn't, perhaps, the right way to go about it. I would argue, even if the argument is off my gut instinct rather than figures, that the major logical push for monogamy is rooted
within the emotional side of it. The way you feel about it is reason enough for the idea to be scrapped, I would think.

So I don't think your feelings are unfair or archaic or what-have-you. They're pretty natural.
I would tell you as long as she values that loyalty to you as she does, you've nothing to worry about. Fantasies are just that: fantasies.

Heck...lol I would think you two, if you were so inclined, could maybe explore it without the involvement of other people. Be creative.
Anyway, I hope I was of some help, and I hope you'll be able to find some peace in your time here.
Hugs,
Harley