I hear a lot of 'this is what I know about poly and he doesn't know as much as me' going on. I'm not sure that's helpful. I would suggest you both doing research, there's lists here of good books to read, threads to check out, blogs, and most of these are really just guidelines. It's really up to the people in the relationship to figure it out best for themselves. Really it boils down to what you need or are want out of a relationship. You say you don't want a serious relationship with him, seeing him once a month is fine, so that's your expectations, and that is discussed. When those things change you discuss them again.
I am one of those people that was married first, and then came into poly but believe me it's not the majority or even the most 'popular' way. Everyone has a different style because everyone is different!
Yes, when BF and I were having a hard time, it was felt in my marriage. Felt by the kids too. They would give me hugs and say sorry and they would tell me to let BF know they still love him. DH didn't ask details because he knew it was an issue for us to work out, I gave an overview, communication issue, and he sent some reading materials that helped him to BF hoping it would help.
Now this is pretty much what would have happened if it was a good friend from work or socially too, so the fact that it was a romantic relationship did not make it WORSE. One of the things DH does that is amazing to me is that even in the beginning, when things were hard for us and we were stumbling into this, he would ask himself, "How would *I* want to be treated if I was the other person?"
I say it's amazing because though he and current BF have quite a bit in common, are not friends but have mutual respect for each other, it was not always the case. There were people I was seeing that he had NO respect for and for good reason. Yet, he worked on what upset him and why and how to deal with it, and still asked himself, "If it was me, how would I want to be treated?" It's not easy, and so many people don't think of that question even in daily life, but he does and it's not only helped us all in this but still blows me away at how unselfish and amazingly caring he is as a man that he would ask that in situations where he would be more than right in thinking only of himself.
Ask that question. Not just of this potential partner but of HIS potential partners. If you had a bad day at work, or a problem with a good friend wouldn't you WANT to be able to be yourself, feel down and have your partner hold you or at least express regret at you feeling badly? Or would you want your partner to tell you to deal with it on your own time and give him your undivided attention or you aren't worth his time?
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year