Could slow this down and organize the many problem layers in order radiating out from you, then to you+partner, then the family, then the town, etc. You have a lot going on there all mixed up.
Could not jump into polyshipping before sorting out your other things first-- like the unstated standard for the relationship that he broke somehow by watching porn. What relationship standard was it? Was he aware of it? Or was it unstated expectations? (Not a standard if he did not agree/sign off to up hold that standard.)
If your 2 people relationship is not up to standard, how can you run a 3 or 4 people polyship to standard? Could work on communication skills a bit first.
What need was not met at home? Were you needing more support in a "dealing with health issues" time of your life that you did not get? Did you ask for support from him and how you wanted him to provide it?
Because to me? Right now you are two people in a relationship where you are upset over needs not met triggered by watching some TV media thing. If you stumble there? Not able to articulate your needs so they could be met and making it be about the trigger (porn watching)?
Your intrapersonal and interpersonal skills can't hack relating to 1 partner? How can you hack relating to 2 partners, or 2 partners and a metamour?
Because the TV can be turned off and neglected for a while as you sort yourselves out when it's just 2 of you and some TV trigger.
When it becomes a polymath
problem with more people on the line? That's a whole other kettle of fish. People cannot be "turned off" and neglected and spring right back up how you left it when you want it again like a TV can.
If you have cancer grief from uncle, other upsets from PCOS/surgeries... that's already two things to have to sort and process inside yourself before entering healthy polyshipping. 2 bags. Then you have this relationship standard thing with the partner to sort out with him. 3rd bag.
It's fine to be atheist and not believe in any God(s)... but how do you tend to your spirit as an atheist? You do tend it right? Even if you do not subscribe to any notions of God(s) or organized religion you still have a soul to care for. So are you in good spiritual health? Because poly can be intense. If you are not in good spirits and you enter polyshipping in low in spiritual health? Poly can ding you up some more. Maybe you have a 4th bag then -- with maintaining spiritual health as an atheist. Just the uncle cancer and your own health things -- those can drain you in your spirit / spiritual health bucket.
In short -- you do not sound prepared for poly. Could prepare more to avoid potential pitfalls.
I'm not saying "do not do it." Just saying "sort your other stuff out so you can do it WELL." Enter into polyshipping with less baggage weighing you down. Increase the odds for success -- YKWIM?