It may be
Quiet Summer by Sotia.
In this fic, Will is younger than Buffy.
A night, at a club, he sees her hugging Riley, her first boyfriend.
He gets angry, go all caveman, bring her home and take her rough, right on the kitchen counter, all dominant like.
It appears that Riley is now gay, and dating Parker.
Quiet Summer by Sotia
Summary: 23 year-old Buffy has to spend her summer with her teenage cousin Xander and his cousin Will... who's no longer the shy boy she remembers. Or is he?--------------------------------