Glad to hear that he's cool with you guys cuddling and being lovey, you dont wanna feel like you have to be weirdly formal with a partner.
That said, I sympathize with both you and Fly on this one. Of course you never want to make your gf feel like she's required to participate in threesomes if she actually wants couple time -- ew, that sounds unpleasant for all involved -- but the idea of being in the house with my partner and her other partner, my lover, while they make love without me kind of kills me. I'm just not quite there with my compersion, though I think I'm much closer to being cool with it then I was.
G&E&I have navigated this by the one not involved leaving the house for an hour or so if the other two are having a date night but aren't going out. Either he or I, depending on who's having the date, will take the baby somewhere, or go on a shopping trip, or hit the diner. It's a matter of giving the other two privacy for couple time, and of not having to wonder what's happening in the other room, or if you're going to hear something and get envious, or whether you're going to be invited in, etc etc. Plus, it's an opportunity to get things done, visit friends.
I really am anticipating that we won't need this coping mechanism soon -- once I came home while G&E were still in the shower together and I was totally ok, and the other night Eric got home while Gia and I were still in the bedroom together and he was chill (and even joined us when we asked
). It's just been one of those surprising poly mental/emotional blocks that seems to take time to break down. As the hinge of your vee you don't have to worry about it so much, since they're not clambering to be alone without you, from the sound of it.
Anyways, just some thoughts!