It means that people who are married usually mention it fairly often, and the Giant never mentioned being married. In fact, his oblique references to someone caring for him when he's ill made me think that he wasn't married, since otherwise, why not just say "my wife takes care of me when I'm ill" rather than the nonspecific "somebody". So I'm kind of startled that Mrs. Giant appeared after being totally invisible for many, many years.
To tell the truth, I've been thinking of the Giant as gay (in the non-perjorative sense) since I first started reading this comic many years ago. I was about 100% certain of it until earlier today when I made my first post in this thread, in fact. Hence my momentary surprise.