It's plain discrimination. Yes, it's the same as if he said I only want to date Black women, or Asian women, or skinny women. But this kind of thing happens all the time. It's a shitty situation, just like getting too fat for a partner who likes skinny people sucks. It can really feel like a punishment when you make an effort to relax your preferences and your partner does not, or even tightens theirs. Of course it does. How can it not? Especially when this preference specifically affects you, and potentially means the end of your relationship.
In my opinion, poly creates the opportunity to relax your standards and be less picky. If I could speak with your partner, OP, I would say, "Come on, man. Why would you need to break up with her? Just because she's mono? Because you need someone who is poly? Why would you do that when you can have someone who is poly AND someone who is mono?" Sorry, but this is a no-brainer.
But some people don't see it that way. You always have to commodify relationships to some extent. It can't be helped. But there is such a thing as too much commodification. It sounds like your partner wants to max out the awesomeness of his partners, no matter if he has one or two or three. There is no room for someone who is great in this department and lacking in others, and it doesn't matter if the lack can be made up by another partner. He wants partners who are awesome in all departments, all the time, all the things! He wants poly women and has no time for you. No room for unique and special snowflakes, only ideal snowflakes with the means to serve his ends (which don't really exist).
Okay, I'm dramatizing this a bit. Maybe this is a worst-case scenario. I don't know if I would date someone like that. Sure, my partner has preferences. I am dating someone new right now who has a no-kissing rule. My partner says he could not do that. Kissing is a must. He would not date her. Me, on the other hand? I don't see a problem. I can kiss other people. I don't need to kiss her if she doesn't want to, and we do other fun stuff together. If I really want to, I CAN JUST GO FIND SOMEONE ELSE TO KISS AND ENJOY WHAT WE HAVE. Is this so difficult of a concept? I don't think so.
*shrug*
In my opinion, poly creates the opportunity to relax your standards and be less picky. If I could speak with your partner, OP, I would say, "Come on, man. Why would you need to break up with her? Just because she's mono? Because you need someone who is poly? Why would you do that when you can have someone who is poly AND someone who is mono?" Sorry, but this is a no-brainer.
But some people don't see it that way. You always have to commodify relationships to some extent. It can't be helped. But there is such a thing as too much commodification. It sounds like your partner wants to max out the awesomeness of his partners, no matter if he has one or two or three. There is no room for someone who is great in this department and lacking in others, and it doesn't matter if the lack can be made up by another partner. He wants partners who are awesome in all departments, all the time, all the things! He wants poly women and has no time for you. No room for unique and special snowflakes, only ideal snowflakes with the means to serve his ends (which don't really exist).
Okay, I'm dramatizing this a bit. Maybe this is a worst-case scenario. I don't know if I would date someone like that. Sure, my partner has preferences. I am dating someone new right now who has a no-kissing rule. My partner says he could not do that. Kissing is a must. He would not date her. Me, on the other hand? I don't see a problem. I can kiss other people. I don't need to kiss her if she doesn't want to, and we do other fun stuff together. If I really want to, I CAN JUST GO FIND SOMEONE ELSE TO KISS AND ENJOY WHAT WE HAVE. Is this so difficult of a concept? I don't think so.
*shrug*