Hello, polyamorous person here. Of course, I can only actually marry one of my partners because of the laws here, at least legally, but hey.
Let's face it, if it weren't natural then people wouldn't cheat. The vast majority of people will cheat at some point in their lives. Therefore...why the fuck not? Allowing each other to do whatever the hell we want as long as we're careful, vs. pressuring each other to never even LOOK at another person again which will probably make everyone involved miserable and drive a wedge between us - easy choice.
I would love to hear about how things work in as much detail as you would like to give. It kind of sounds like you have an open relationship as opposed to being married/"married" to more than one person.
When I lived in Spain, some of the Arab immigrants I knew tried to pull that. In their countries is fine, but in mine is not, anyhow, it was somehow common to know a man that had bring his wife with him and tried to get a second one there. It is shocking if you are not used to it
I have to be honest, I am not going to lie, it grosses me out big time, I would not accept that willingly, not for me, but what people does with their lives is their business, as far as does not affect me directly