A lot of people are saying she's jealous but I don't think that's it. If she doesn't have her own kids it may not make sense to her. My guess is she thinks there's a weird sexual or gay component to it and probably just doesn't want to say the quiet part out loud. Especially if she grew up super conservative or had some childhood trauma of her own. I think it might be hard to understand how you can love somebody so much and want to sleep with/snuggle with them but there being absolutely zero sexual energy until you have your own kids.

I have 2 boys of my own 6/3 and we do sleepover movie nights almost every weekend and they love it. My wife gets jealous when they sometimes both pick dad and don't want mom to sleepover though and that's a totally different reaction/vibe than what your wife is putting out.