Ah, yes. You can't have your cake and eat it too. Boy, did I hear that line a few times back when my husband and I nearly separated over this issue. Though I was terrified to lose him, I didn't back down when he resisted. I made it clear that this was important enough to me that I would walk out of our marriage if I had to, and if he couldn't work with me towards a new way, we were doomed. It wasn't exactly an ultimatum, but I did make it clear 1)that I loved him and wanted to stay in the marriage, but 2)I was unlikely to be happy in the marriage without some measure of openness.
I was so frustrated by how many people automatically took his side and said I was supposed to suppress such desires. Or others that said I should scrap the marriage and go search for some hypothetical partner(s) who wouldn't expect monogamy. I was faced with a lose-lose situation: leave my loving marriage, or give up on being truly fulfilled relationship-wise.
Luckily, my husband did some soul-searching, and agreed to give it a shot. Since then, we've BOTH had to make compromises and step out of our comfort zones. It is certainly not perfect: I'd like LESS rules and more openness, and he'd love for me to just forget the whole thing and decide monogamy is the way to go. But, our marriage feels stronger than ever, we communicate more and fight less, and there is no longer talk of separation.
Stay honest with yourself and your husband, and hopefully the path will reveal itself. We're rooting for you!
Early 40's female, bisexual.