Hey DaniellaWidget,
It's 'Something Real' By dampersandspoons (now Grief Counseling on Sunnydale After Dark website). This particular story is still available on Nocturnal Light.
By chapters and here's the complete story
on a single page, in case you want to save it.
Summary: Buffy is a single woman empowered by her sexuality yet detached from human connection She's never had a real relationship, never slept with the same guy twice, and never had any intentions of doing either until she goes to a party in down town NY and meets a mystery man who changes everything. This is an angsty ride, so please read the A/N's on Chapter 1. (This is based on the film Lie With Me but with my own twists and turns.)