There should not be a committed, long-term relationship between any D&D PC and an NPC who takes an NPC class or is 2 or more levels below the PC.I just had a character get married to an NPC in one of my games; I allowed it because not allowing it would have gone against setting and characters. Now, I am trying to write a balanced (read: dead-weight but not completely useless) NPC for that and was contemplating the consequences of the relationship for the character and the game.
Seriously. Such things should be flings. Flings that can be killed, make the PC feel bad, motivate the PC to hunt down the killer, and then encourage her to find someone more on his level.
And when I say level, I'm serious. Having a regular, ol' commoner ball-and-chain is not a role-playing hook--it's the ball-and-chain's death. The adventurer's lifespan is measured in months. His enemies should be constantly looking for a way to hurt her. To have a commoner spouse (especially with the standard array when the PC routinely encounters beings with elite arrays) when the PC's a Wiz15 or whatever is like posting a sign on the spouse's back that says Kill Me.
However, if the person is in a position of power (usually, in NPC class terms, possessed of great wealth), then the spouse can afford protection while the PC's away, but there's nothing the PC can do if she's a Wiz15 and he's a Com1, except hope that he really likes tiny extradimensional spaces, because he's spending the rest of his married life in a fucking rope trick.