> Why would something become acceptable/inacceptable simply because of the gender/religion/haircolour/height/taste in music of the recipient?

There are some things that might (for instance, a reference that had a widely accepted meaning in general usage but also a specific use in the context of a particular group might be less appropriate to use to a member of that group where the context might be ambiguous enough to support both the group-specific and the more general use, while being perfectly fine otherwise), but I don't really see that being the case for the particular email in question.