I don't think it's about Buffy and Angel kissing as much as it's about Buffy betraying them. Buffy was out of line when she accused him of jealousy and spying on her, did she forget all the horrible things Angel did last year? If she could, the others can't. Angel tried to kill Buffy's friends to hurt her. Willow in Innocence, Xander in Bewitched Bothered and Bewildered, and Giles in Becoming. She can't expect them to be understanding and welcoming when they discovered she was hiding the "murderer" for a while now, to the point where she's so comfortable with his presence to kiss him.
Xander was angry at Buffy for hiding Angel and then being stupid enough to kiss him and give him another happy. It wasn't about jealousy, it was about betrayal, and at that moment he felt betrayed.
Buffy: (hesitatingly) Are we cool?
Xander: Yeah! Just seeing the two of you kissing after everything that happened, I leaned toward the postal. (pauses) But I trust you.
If it was about jealousy, Xander would have objected to Buffy's relationship with Scott, but he didn't. During S3, he was more into Cordelia and Willow; the only moment he appeared to want Buffy was when she read his thoughts about "sex with Buffy".