My girlfriend wanted a 3some with me n anotha guy I rejected the idea is she ready to settle down with me only

I neva agreed with her doin it, at first she said she only thought of it cause I mentioned it 2 months prior, about her, anotha girl n myself, then she later on said that she'd been thinkin about it since she was in high school, she's now 20 and I'm 21. she says she's ready to marry me and start a family real soon, but I wounder if it's fantasies in the back of her head that she will always think about, and if she's ready to settle down. if she goes thru with this I will never look at her the same, and we won't be 2gether. So what I'm gettin at is do she still have ual thoughts that needs to be fufilled without me in the picture or stay with me, and I don't want to make her seem like she have to pick, cause I know she'a choose me. I jus just wanna know will this urge grow so strong that she'll eventually do it with or without me? and keep in mind that she refused to do the 3some with anotha girl because "she don't want to share me with anotha girl" but she wanted to share herself.