I have been in a serious relationship with my partner for almost 6 months now and I was very upfront with him about being poly and married. I have started to have serious problems with my marriage that may end in divorce but now my partner is suddenly asking me to not be poly anymore if I get divorced, he says that he realized that he hates sharing me and he couldn't handle me being with anyone else. I'm not sure how to handle this, I'm fine with mono relationships but he's being pushy about it.
If you would be happy dating this guy monogamously, then you are well within your rights to decide to shift the terms of the relationship like that! You were polyamorous when it worked best for you and your relationship, and monogamous under the same circumstances.
But if you don’t want to date that guy monogamously, then you don’t have to! You can tell him that you are not interested in changing that term of the relationship, and if he can’t stay in a relationship with you on those terms, then you’ll have to sadly part ways. Only you can decide!
However, it sounds like you’re more worried about his pushiness and the way he’s framing this change. Even if you could theoretically be happy in a monogamous relationship, that doesn’t mean you’re obligated to try it right now, right here, with this guy. If you don’t like the way he handled the request, that’s a completely valid reason to turn down the request. If he made a reasonable request in an unreasonable way, it doesn’t really matter that the request was initially reasonable.
If he’s generally behaving in a way that feels pushy (a word you used) or even possessive or controlling (words I acknowledge you did not use), then you can reconsider the terms of the relationship on that basis alone. Have you ever told him “no” before? Is this part of a larger pattern? Note that 6 months isn’t a whole lot of time to date someone, especially since at least some of that time has been shared with the emotional turmoil of your marriage struggling. Note also that some of the language you used when quoting him sounds very, well, controlling and possessive.
I’m not usually one to go the “trust your gut” route, but if you don’t like how he’s being with this, I think you should take that as a strong signal. My advice is to hold firm that the relationship is and will remain a polyamorous one, and if he’s not okay with that, it will need to end. If he tries to argue or push back or demand monogamy or otherwise not take no for an answer, that is NOT a sign that you should have gone monogamous, it’s a sign that you should LEAVE the relationship rather than continuing to negotiate for it.