Name changed for obvious reasons.
Background is relevant here - we are both 26, been together since we were 21. Both (as far as I know) been faithful. We were recently watching Easy on Netflix and there’s a married couple on there exploring polyamory. Bf asked me how I felt about that, and I stupidly said I didn’t think I’d be overly jealous. I don’t know why I said this, I think I just don’t know my own feelings or something. I said ‘hmm do it if you want’ but I didn’t think he actually would - anyway next day I discover him swiping frantically on Tinder, Bumble and Ok Cupid - he has downloaded all three and has set his age settings to 18-21! An 18 year old girl would have been like 12 when we met, which just seems like a very big and worrying age gap. Also I feel like it’s predatory. He showed me the messages he’s been sending as he said he has nothing to hide since “we agreed” and I felt overwhelmed and upset by them, sending a message to a girl carrying a cat which said ‘nice pussy’ - he said he’d not get emotionally attached and that I’d always be his no.1 
I have brought this on myself I know by saying I found the idea interesting and even a sight turn on when we were just talking about it. I’m not happy about this anymore though and don’t know what to do.
Aaahhhhhb