My partner is interested in polyamory, but I am not - what does that mean for our future?

My partner mentioned potentially being polygamous but I don't feel comfortable with any third person. I want to support them but I'm too possessive to share. I haven't really told them how I feel about this yet though. How do I properly tell them and would that ruin the relationship? Is it necessary for a long term relationship to have multiple partners? Or is it more something they wouldn't mind either way? I'm uneducated and scared help.

First off, you’re going to have more luck with the term “polyamorous” than “polygamous” when it comes to Google searches and reaching out for support.

I have no idea whether telling your partner this information would “ruin the relationship,” because I’m not psychic and I don’t know your partner. If they want to be in a relationship where polyamory is at least an option on the table or where their partner is willing to entertain the possibility, and you tell them that it’s absolutely not, then they might act on that information and leave the relationship. You didn’t “ruin” anything, you just worked with your partner to figure out whether continuing the relationship is going to be the best call. If they were just vaguely curious about it, and happy to drop the issue permanently after hearing that you are not at all interested, then that’s another story. I can’t predict that with any accuracy.

Of course having multiple partners is not necessary for a long term relationship in general; lots of people have happy, healthy, long-term monogamous relationships. I’m sure you know a few! But the issue isn’t whether it’s necessary for everyone; the question is whether it’s necessary for your partner. If you are a person who can only see themselves in a long-term relationship with someone who would never consider polyamory, and your partner feels differently, this specific relationship might not be sustainable long-term. But that’s okay! That’s why relationships start out as short, so you can figure out whether you want to keep them going! You and your partner are doing everything right, by identifying your thoughts and feelings, then being honest with yourselves. The next step is to be honest with each other, then act on that honest information, not withhold it for fear of what it might mean.