That's a tough call. I don't know if that's normal or not; I haven't been fortunate enough to find a long-term partner. Open or polyamorous relationships often bring extra challenges; any machine is more prone to breaking the more moving parts are added. Is it them specifically that you find boring, or is it what the two of you are doing that's not meeting your needs? If you're not switching things up with each other every now and again, I can see things getting boring that way.

From a pure logistics perspective, I'd suggest talking with your partner and seeing if you can work together to come up with ways to alleviate the boredom you have with the relationship before opening it up to others. They might be bored in the bedroom too. I'm not qualified to say what's wrong or right in your case, but an open relationship, that's a knife that can cut both ways.

There is the Swingers scene, you might be able to convince them to give that a try with you, but I've never experienced it and have no clue what's to be expected.

Whatever route you take, I hope you can solve this to your satisfaction. Please let us know how things go.