I'd be angry too if someone would rather make out with a walking corpse than with me. Moreso, a soulless walking corpse. Talk about a slap in the face.
If you're talking about Angel, he wasn't soulless, and, to the best of my knowledge, Buffy never wanted to "make out" with Angelus.
Anya was the one who made out with the "soulless walking corpse" (who had been recently told by Buffy that she didn't want any sort of relationship with him ) but only after Xander had left her at the alter, signifying to any and all that he released all claim on her, and in defense of both girl, (more accurately, at that point , fully restored and back in business "vengeance demon,") and the "soulless walking corpse," a fair amount of alcohol had been consumed by both parties, neither of whom was, by any definition of the term unattached to begin with. But since you brought that incident up...
Xander's unjustified jealousy isn't just limited to Angel and his relationship with Buffy. He was also jealous of Wesley and "his" Cordelia after they had broken up and he had no claim to her affections, as well as the above mentioned case with Anya and Spike. Apparently, Xander is of the opinion that, once he shows interest in a girl, whether she currently returns that interest or not, she is his property and his alone, off limits to anyone else. Riley was exempt from Xander's wrath, most likely, simply because of admiration bordering on hero worship, and probably the only thing that saved Wood in Season 7 (who showed interest in both Buffy AND Faith) was that Xander was just too occupied with having repeated bouts of "break up sex" with Anya to notice.
Each character in the Buffyverse seems to have his or her own unique Achilles heel, and unwarranted jealousy seems to be Xander's.