Advice needed

Me and my partner have been together for a few years and have recently started to talk about adding another woman for a threesome. I thought it was hot at first and thought i would be super into it, and part of me still feels like that however i feel like its kinda taken over and that’s brining up some emotions in me that I didn’t have before, like confusion, sadness and insecurity.
He’s been talking about it a lot, like it’s only been a few weeks and he’s brought it up a lot more then I’d like to hear and i asked him why and he said that he would find it hot and it’s that sort of thing you want to do more since it’s kinda risky in a way and that he just wants to try it, and that he would find it hot seeing me with another woman. I feel like it started from that point but now it’s slowly turning into him wanting to be more involved and I just think it’s going way too far. I’ve mentioned to him like he would probably feel a little uncomfortable if it was us wanting to bring another guy and I kept mentioning it. I did bring up the idea of another couple being involved and he then went on this rant of how he’s so confident in himself in the bedroom that he wouldn’t care if it was a couple, and honestly that made me sick hearing that bc like the over the top confidence isn’t something I want to hear in this context.
The other night we went out, and a girl asked to kiss me and I said yes and he saw of course but this had already been established that this is okay if it’s me and another girl, and that was fine that night but he brought it up again saying how he wants things to be fair so whatever I’m doing he should get to do, but I don’t think him kissing another girl at the bar is the same as me, I could be being bias but i just feel like that’s a boundary I’m not comfortable crossing unless it’s like in a safe space where the actual act may happen, just not in a public area like that. I also feel like that makes it lose the purpose of this originally being a thing he wants to watch.
I would really appreciate some advice as this is a new thing and even if I may be in the wrong I would appreciate any advice!! Has anything similar happened to anyone else? How did it impact your relationship?