1 - Me and my partner are younger, but she has already been told about my poly and such. She took it quite hard, because it was something new to her. You say your partner said it was a good idea.
2 - Same sex relationships are only different in anatomy. My partner is bisexual. Being comfortable with someone shouldn't offend them at all. It should make them feel a lot closer to you.
3 - I think monogamy is more natural in the sense that it is the normal thing for people to do these days. Although I don't agree it should be. But people have their own choices. If your partner finds others and you stop being comfortable with it, well that certainly is a shame, but you can't stop it. Nor should you try. But perhaps if that came up, you should talk to her about it and maybe she could make you get your comfort back. If not, then surely it wouldn't be a desirable relationship to stay in. But you could at least salvage a great friendship.
Hope I was any help at all.
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