If you think about it, many novels (except, maybe, ones that are specifically focusing on family life) have protagonists with no significant other.
Why is that? I think it’s because husbands or wives or--in the case of children’s lit, parents--can be domineering, interfering, or overly influential. Worst case scenario, they can weaken the protagonist as a dominant force in the story.
I was editing my current WIP yesterday and noticed that my protagonist is a widow. Not only that, but four supporting characters are spouse-free.
I’m fine with my sleuth being single (her husband might try to curtail her crime-fighting activities: particularly after they put her in danger), but I’m going to take another look at the bevy of single characters in the supporting cast.
I always thought it was funny in Cheers that Norm’s wife became a character, even though we never meet her. Just his remarks about her were enough to bring her to life.
I think the trick is to have these characters fleshed out and represent them as part of a family—even if it’s one that doesn’t mind if they’re out at one a.m., fighting crime.