Personally, I do believe monogamy is toxic by being an extension of centuries of toxic cultures having influenced human society conditioning us against polyamory. Humans historically practiced polygamy until arbitrary religious and socioeconomic beliefs began to influence our society, teaching us to be ashamed of our natural behaviors.

But that is much deeper issue with society at large which is neither here nor there.

If someone doesn’t like their partner doing something that is harmless and not affecting them in the slightest, then they need to practice some introspection into why the actions of others that do not directly affect them or harm anyone else bothers them so much and sort that out for themselves to become emotionally mature instead of trying to control the behaviors of others.

Also, my logic makes allowance for monogamy. In a monogamous relationship, going outside of it as one would in a poly relationship comes with the risk of STDs, which directly affects the other partner, so it makes it their business who their partner sleeps with to be able to have informed consent if they want to continue being in a sexual relationship with the individual.