PinkDragon has a lot of good questions.
Really, you may not have even noticed you didn't have as much of a sexual connection before you started seeing somebody else, we get oblivious sometimes. What sorts of things do you think help keep you from wanting to have sex with your husband? I know lots of times I may be in the mood and if Adam says or does the wrong thing (which for me happens to be totally oblivious that I want to have sex with him) my interest in being sexual can disappear in the blink of an eye. So I wonder if you do want to sometimes and chores/life/problems douse your libido, or if you just never even think of him as a sexual option
Obviously if it's the latter, maybe you do want to think about if this relationship is right for you and you were looking at poly as a way to get something you weren't getting in your marriage without trying to nurture it there first. I notice in your post you seem to have gone right to "you deserve better than me" instead of "hmm how can I reignite the spark (assuming you did have a spark) we used to have"
Sex with a non live in partner can stay great for longer I think - there are no chores, there are no arguments about bills (generally) and so a relationship can seem almost perfect. Are you letting that make you compare your sex life with your husband with your new partner? Do you find it pointless to have sex with your husband because it comes with the baggage of 7 years? (I've been with my husband 8 years, for perspective - if he was interested, I'd have sex with him 5-6 days a week, about as frequently as I wanted to when I met him.)
So does it bother you, or him that you're not having sex?
If so, do you think it's lazy to not be working on having sex with your husband and letting another relationship distract you?
Are you being passive aggressive by not working on that part of your relationship and then projecting on him that he must want to leave you for a mono partner, so the onus is on him to tell you if he's not happy with the platonic roommate thing? Too scared to admit it if you're the one who is not interested in staying together?
Why the hell are you not making time for each other? Stop taking each other for granted ok?
Laziness in regards to relationships don't work so well in poly
Obviously we don't know if you'd be better off as just friends. But if you're not working for something better than that, that's all you're going to get out of your relationship.