I think that the biggest issue here comes down to consent. In all sexual relations, consent should be obtained by all parties. And whether or not you realize it, your boyfriend is involved, he is not consenting and he will get hurt Your actions are putting him unknowingly at risk for not only mental harm, but also potentially putting his sexual health in harms way too.
Not everyone is cut out for poly, so it isn't fair to put someone in a 'poly' (or cheating) relationship with out their consent.
I have faith that if you talk to your boyfriend about it, things will work out, whether that means breaking up or exploring consenting poly dynamics in your relationship together.