there are several stories like this but in this one, Xander confronted Buufy and Spike is his crypt. He intended to stake Spike but Bufffy stepped in the way and he stabbed her instead.

Later at the hospital, Xander tries to hide from the Scoobies what he did but Buufy explodes. One of her lines was something like, "Why you and Giles think you have a right to regulate my vagina..."

I used Google to look up that phrase about access to her vagina and found Road Full Of Promise by Lady Sanume on ff.net: https://m.fanfiction.net/s/10341737/Road-Full-of-Promise

That's the one! Thanks so much!