Honestly I think the resentment (which is valid) is coming from the fact that Flo is doing things with this woman that you wish he was doing with you, and he's not.

Why don't you guys take each other to a lake? Not "instead of" the other woman, but *as well as*. A special time for just the two of you?

Why don't you make special efforts like this for each other? Instead of only spending this energy on new partners?

I'm not saying don't be poly, that's a choice that you feel strongly is right for you, but I'm struggling to see why it's only the new partner that gets the special attention that really should be going to you as well.