Husband wants open relationship

My husband and I got married about 1.5 yrs ago. We had been dating for 2 yrs and lived together 6 months
Things have been Rocky since we got married but it’s mostly over finances, his temper is a bit hot so about him not flying off the handle, me being more patient and less selfish, and housework. Kind of normal things. We put the work in to try and make things better and got to a better place where we’re looking at our future together.
I’m a bit upset because lately he has been bringing up this idea of an open relationship. He has always said how he wants a three some but I honestly thought it was a joke that all guys want that and I always was adamant that I would never participate in one.
He brought up seruosuly just now that he actually wants to “set rules” and “explore” more into this idea of having a threesome. He wants me to find another girl for us or vice versa. I’m honestly disgusted at this idea and I only want one sexual partner at a time especially if I’m married. I didn’t know what to do when he brought it up so I just said okay I’m not sure I’m interested in something like that. And he said I think you are b it but you’re afraid because it’s a typical.
I’m very upset and think this is something he probably won’t let go. I feel I deserve someone who only wants to be with me and not other people and I’m debating walking away. I’m 29 and have no kids w him; and I just feel there must be someone out there who doesn’t think this way.
What would you do? Please help!