It's interesting that Sara would have supported Xander marrying Anya, because I'm firmly convinced based on how Anya was portrayed, even in her flashback human days, that she had Asperger's syndrome, like some people have suggested Gil Grissom has. For those not in the know, Asperger's syndrome is an autism-spectrum disorder that primarily manifests in problems with social interaction; those with the disorder are often very literally-minded and extremely intelligence, but have little understanding of social nuances. I speak with some authority on the fact, as I've suffered from the disorder for 35 years but was diagnosed less than a decade ago. For example, the reaction Anya had in "The Body" after Joyce died? Very plausible for someone with Asperger's, especially if it was the first someone she liked died. We have a very hard time censoring our emotional reactions to stressful events like the deaths of friends and family.