Partner of 5 years has spoken about threesomes/couple ‘fun’, I am scared to venture into this but am also curious

So for context me and my BF have been together for 5 years and in the first 3 or so years we were quite happy sexually and though there’s been some issues on his side with performance anxiety, I was always happy with the foreplay stuff.

Last year he slowly introduced the idea of us doing something with another guy or maybe a couple swap. Though I have fantasised about it, I’d never wanted to go as far as to actually do it. I really value our relationship and I’ve read a lot of stories of relationships never recovering from threesomes and similar scenarios. I really fear that happening with my relationship. I really don’t want to risk my relationship for sexual endeavours.

The problem is, I do feel now our sex life is somewhat vanilla because there’s just this unspoken thing of not venturing into something he clearly wants to do. Also prior to being together neither of us exactly had a colourful or experienced sexual past. So there is that element of should we catch up for lost time so to speak.

I just wondered if anyone else here had been in this 50/50 scenario. It’s like I want to but I really really don’t. By doing so I feel I’d be cresting problems I mentally can’t take at the moment.