i read a story long, long time ago, they meet as vampire and slayer at a Laundromat and i remember that buffy has to borrow his coat while she washes her clothes and they're not wearing a lot of clothing... because all their clothes are in the dryer. they may have sex later on in the story,

Sounds like something from The Butterfly Effect by cousinjean

Morvern
It's definitely The Butterfly Effect. I just read it a few days ago.

No, i went through the Butterfly effect twice, it's not that story. I believe the story im looking for is only one scene.