I'm a newbie here with no "creds", so...warning!
That said, the solution to "our sex is not satisfying and infrequent" (and at least I think messages in your post that you view this as "her issue") of extending to an additional relationship (polyamory) strikes me as superficial.
I'd suggest you as individuals have some significant growth to do that can result in a transformation in your relationship and all attributes of your sex life. That could still happen if you enter into an additional relationship...but it sounds more like using the additional relationship to AVOID that (which assumes you know it's a very viable path, which you probably don't hence your post).
I was in the same situation and did the "cheating thing" for a very long time. I also thought it was "her issue" and she wasn't sexually inclined the way I was. Boy was I wrong. She had issues...I had issues...we had issues together. Result of working together on them with professional help: an exceptional relationship in general and incredible and very frequent and satisfying sex! And we aren't close to done yet, either (probably never are!).
I strongly recommend "Passionate Marriage" as a low cost way to get into the complexity of relationship and sex and issues. Not easy reading but eye opening and powerful!