Personally, I think the group's chemistry is usually improved if there's a web of bonds between the characters--rather than just a bunch of random people who met in a bar. When creating characters, I always encourage the players to think about relationships--maybe two of the characters are siblings, or grew up in the same village, or apprenticed together, or whatever.
Assuming marriage (like any other in-game relationship) is handled in a reasonably grown-up way, I can only see it as a plus. It injects motivation, backstory, and depth into the game--not to mention potential plot hooks.