"anal" can also mean "anal-retentive."

Can, yes. Should, no.
Descriptively, it is used to mean that. However, it is a bad use, because it is ambiguous with its exact opposite.

Should not? Who says it should not? Language is full of words with more than one meaning. I would say that the meaning that means the opposite may have fallen out of use and is irrelevant. I've seen the word anal used mostly two ways: as an abbreviation for anal-retentiveness and as an abbreviation for anal sex. Of course, let's not forget that the original meaning of anal is situated near the anus, which is something that does get messy and has to be wiped clean.

Language is distinct from jargon, at least inasmuch as when the general dialect changes the meaning of the word, the jargon meaning remains just as real. "Anal" has a jargon meaning, which makes the casual use to mean "anal-retentive" confusing.

Anal can refer to any of anal-retentive's. Where I come from, "anal-retentive" means "bad breath" ;)