In review at http://codereview.musicbrainz.org/r/2298/

This has now been merged to the beta branch. As this code only contains database constraints, it can't be beta tested, thus I am closing this.

Why does the artist still appear to have a gender set in the "Artist Information" sidebar?

Good question. Looking at the database directly, that artist's gender is set to NULL, as it should – I suspect caching may be involved.

Yes, it was the cache. I've manually cleared the cache for this artist; as things get edited this sort of thing will disappear.