Funny, I was about to post a very similar thread, with a different contention.
My argument is that Xander is very hypocritical when it comes to his disapproval of Spike.
It's highlighted for me, most profoundly in the episode "Entropy." He says something about being disgusted by Anya because she let that "soulless thing" touch her. But let's compare chipped Spike and newly human Anya. I think they are quite similar and if anything, Anya is worse, (though I adore her as well.)
Why? Anya chose, while human, to become a demon and to slaughter and torture countless men. Spike, while human, did not chose to become an evil, soulless killer. Then, Anya chose to become a killer once again, in season 7. At the same time, Spike had just come back from seeking out his Soul.
So, Xander is totally fine with the idea of dating and sleeping with a woman who "maimed and mangled" for over 1000 years and then decided, unprovoked, to do so again in recent history. But he disapproves of a man who unintentionally became a killer and who hasn't been able to hurt anyone/ hasn't wanted to hurt anyone for quite some time.
To sum up, when considering Xander's hypocricy, I think it is most poignant when it relates to Anya/Spike. They both had their evil powers stripped away from them; Spike with a chip, Anya with a spell gone unexpectedly, (The Wish). Neither one of them chose to be "give up" their evil tendencies, but there they were. Both of them, for all intents and purposes, newly human. Xander sleeps with one but judges Buffy harshly for sleeping with the other. To quote Xander, "How. What? How." 🙂
(Disclaimer: I am a total Xanya fan.) 🙂