Looking for advice on how to propose a FMF threesome to my gf

Me 26, her 24. Our sex life is good but I want to add something new

I’m curious about a FMF threesome (not looking to open the relationship, just a one-time experience we could explore together or it may depend)

The tricky part: during truth or dare with friends I said I’d be open to two girls, she said she’d want two guys. She got upset thinking I was making a proposal… I wasn’t, just answering the question.

So now I don’t know how to bring it up without triggering that conversation again.

How do you propose this to your partner in a way that feels like an invitation and not a threat?