My girlfriend wants to sleep with a guy before marriage

Like the title says, my girlfriend is bi and has never slept with anyone. She told me before she settles down, she needs to have slept with a girl and guy to know the difference. I feel conflicted with not wanting to tie her down but we've also talked about how much we want to live together and spend our lives together. I talked about this with her and I can't see how she'd have the chance to sleep with a guy if we're still dating??

We've established we only want a monogamous relationship so I dont know if I'm overreacting but it just feels like an insane thing to say and be very decided on while we're dating because it makes our relationship feel temporary?

edit 1: Thanks everyone for your advice😭. I've been reading the comments and I don't want to jump to ending things immediately until we've had a proper conversation about this again. To clarify, she never requested to do anything non-monogamous and assured me she'd never cheat, although when I asked her about how she'd actually experience sleeping with a guy she kind of joked about it without really giving an actual answer. I guess it's difficult to take in that this could be what ends things, but I want to try communicating with her first.