I've played in five groups with GM/SO combinations, none with GM/non-SO relationship.
In three GM/SO groups, the wife of the GM got no preferential treatment, never asked for it.
In one of the other two, she didn't get outright favoratism, but did appear to gain things such as more contacts, information and a couple of things the rest of us would have liked, but never got out of the GM. Unfortunately, she played the Spoiler, deliberately screwing up player tactics and role-play encounters while sitting back, laughing and looking sideways at the people who became upset (usually me, being the tactician). Her boyfriend GM defended her "fun", until it was made clear that four of the five other players were bothered by it.
In the other one, the GM's wife got most of the same perks as Spoiler Chick. Then she got her character in a situation she shouldn't have, and (while in her defense she was 8 months pregnant,) threatened to make his life hell if he killed her PC. In the end, two players (not me) made it known that this was not cool and her character died. Unfortunately, she came back with a 61 point PC that I'm fairly sure wasn't actually rolled. One of the players who objected left shortly thereafter and I left a short while later due to scheduling problems (my new job shift left me unable to attend half of every session).