Sex Life problems

So before my husband six had been bland and pretty much always the same. I’ve had to open up a lot. We’ve done roleplaying, buying new lingerie and sex toys, and doing a few more kinky things like don/sub stuff. But, he’s still caught up because I refuse to try threesomes. My husband before him and I fought for 5 years because I don’t want to try one and now he won’t let it go. No matter what other stuff I say I’m willing to try he’s still hung up on this. Help!!!