M24/F22 – How do I communicate with my girlfriend about a threesome she wants to try?

My girlfriend recently told me she’s interested in trying a threesome with another man. We’ve talked about it briefly in the past, but she wasn’t sure at the time. Now she says she feels more open to the idea.

I’m not against talking about it, but I’m unsure how to navigate the conversation in a healthy way. I want to understand her feelings without shutting her down, but I also want to be honest about my own concerns and boundaries.

For anyone who has dealt with a partner bringing up a new fantasy or desire, how did you approach that conversation?