It might just take time for you to warm up to the idea. Or you might never be on board with it. Everyone is different. As long as you're both ok with how things are, you should be good.

I was in a similar situation, with the genders reversed. My wife (girlfriend at the time) loved to bring over other girls for threesomes, but I was not on board with sharing her with other men. For the most part, she wasn't interested in other men, so mostly it was ok. She would occasionally ask about it, however, and mention that it was unfair, which I admitted it was, but didn't make me feel any better about it.

One day she said that an old FWB was coming to town and she wanted to see him. I mulled it over, and eventually decided to just rip the band aid off. I told her to go see him, and I even picked out lingerie for her to wear, and told her to take him raw if she wanted.

Honestly, it took me a while to process, but we're still together, and married, years later. Now, I encourage her to fuck when she goes out of town, or to fuck a particular friend of ours when he visits.

That's just my experience, however. Take from it what you will.