Well, technically Sasuke was the one who tried something and Neji failed to shove him away and tell him to get lost like he should have. It's still cheating anyway, but Sasuke's the jerk who decided that it wasn't enough to wreck his own relationship and go for a single guy. Nooo, he had to find someone in a relationship.

[She is angry and hurt by what Neji did, but that's mediated in part by how well Neji had otherwise treated her in the relationship - he'd been an attentive and polite boyfriend. Sasuke had always been aloof with her friend Ty Lee and never really made a good impression on Bulla beyond her being able to appreciate that he was a capable athlete.]