My boyfriend, who is 29, wants to explore his sexual fantasy.

Hi Reddit, I'm usually just a lurker, but I'm in a tricky situation and would really appreciate your insights. I'm not the best at storytelling, but here goes... I’m a 30-year-old woman in a relationship with my 29-year-old partner for nearly 8 years. We began our relationship long-distance and still live in different countries. I have more sexual experience than he does, as I was his first partner. A few months back, he brought up his fantasy of threesomes. I was surprised but figured it’s a common desire for many men. When I asked him to elaborate, he said he’d like to have two women (me and another woman) involved with him. Now I'm uncertain about what this means for our relationship. It seems like he either wants to try it with someone else or explore swinging. He argues that we should experience it before getting married so we can focus on us later. While I understand where he’s coming from, as I’m his only partner, I also don't want to hold him back. My brother warned me that exploring these experiences could lead to heartbreak for couples. I think I could handle letting him go to explore his desires, trusting that if we’re meant to be, we will be. However, he doesn’t want to break up. How should I approach this conversation with him?