One is reminded of the Japanese crash quite a number of years ago which was one of the worst examples of CRM (Cockpit Resource Management). Yes, the captain is "in command", but a critical role of the first-officer is to be a second person to check for errors. There was a crew in Japan where the captain was male and the first officer female (not married). Because of cultural issues (especially then), men completely disregarded women and the CVR dialog was something like this:

FO: There's a mountain ahead!
CAPT: No, there isn't!
FO: Sir, there's a mountain ahead!
CAPT: Be quiet!
[sound of crash]

I think that if the couple has a relationship where they view themselves as partners, then they would do an exceptional job of working together in the cockpit. But if it was a relationship where one were dominant, it could very easily negatively affect CRM. Because I don't think the airline can (or should!) know which of these is the case, there ought to be a restriction against this.