Ooops...
Apparently, I thought the body was the title.

When you add a spouse, you have to fill in the last name.
That makes no sense.

This suggestion has been made - may want to search for "Spouse record" and add your votes there.

Also has a comment with info that you can press F3 for name to fill (it's actually listed at bottom of the spouse screen but I hadn't noticed it myself).