No it wasn't. Buffy's a grown ass woman, she's not Xander's properity. She's his best friend, ocassional crush/current crushee. But if Buffy wants to sleep with a dude to feel. Xander's got no say in it. Same for when he yelled at Anya about it. His fault. If he hadn't left her at the Altar for really no reason, it wouldn't have happened.
Xander Harris is a *****ebag. It's funny how he's supposedly the heart of the group, caring, and friends with everyone yet, he's the one whose so quick to judge everyone, but never once stops to look at himself. When he's the worst one of them all in entire group of the Scoobies/Fang Gang.