Just curious, you say you'd be ok with him having a relationship with someone of his same gender -- is he bi?
Ultimately this is a fairly straightforward situation, and it really should be resolved before the wedding. Is he ok with you having a meaningful relationship (not just sex, not just threesomes) with someone who isn't him? Are you ok with not having any other meaningful relationships (beyond sex and threesomes) with anyone but him? If the answer to both questions is "no", there is a fundamental incompatability here and someone is going to have to compromise or else you guys aren't going to work together.
Emotionally manipulating you and another woman into a threesome that he doesn't even want, because that's the only way you and she can see each other, or insisting that somehow you avoid developing feelings for people that you're involved with, when that's obviously not how you work, are NOT solutions. Going back to monogamy, opening up to real non-monogamy, or breaking up are.
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.