Yeah. It is. Like @Lyri said, what makes it all the more sad is that it could have been prevented if Xander had had the nerve to talk to Anya about it before the wedding. Xander made a very bad choice, and Anya was really hurt, understandably...but sleeping with Spike made it so. much. worse...
This episode contains one of my favorite "little" S/B moments, though...so I can't hate it that much. When Buffy and Spike meet in the hallway - the first time since she broke it off with him - they share a moment, and he tells her she "glows"...and the most incredible smile comes across her face, and he smiles back, and it's beautiful. I love the sadness in this scene, but also the way that they are sensitive to one and other and the clear way in which they respect each other, and are compassionate, despite what has happened.
Buffy: But if you're wildly curious, yeah, it hurts.
Spike: But it hurts?
Buffy: Yeah.
Spike: Thanks.
Buffy: You're welcome.
It's clearly hard for her to see him with someone else, and she misses him, and he misses her, but they know better than to talk about it. Instead, they have a honest conversation and are more than cordial with each other - they're warm.