I think a lot of the favortism and such really depends on the maturity of your group and the couple involved as well as how long they've been gaming, either together or separately. I play with a group that consists of three couple and two singles, all of us 30+ in age and most of the couples have been together at least 10 years. The most experienced of us has been gaming for 20+ years and the least experienced 5+.
I do notice the couples sitting together and they'll work on character concepts together sometimes, though often romantic interests are across the party, which leads to much amusement. We have a very good grasp of IC vs OOC, thank you very much. However when it comes to favortism it just doesn't exist. Treasure is split between the party and the GMs, several of us take turns with various games, work hard to create appropriate encounters and rewards for everyone in the party. As well the rules stay constant no matter who is doing something, stupid or not. The current GM's wife, who was playing a high level wizard, put on a cloak without getting it identified first. She expected a cloak of resistance and got a cloak of poison. Instadeath. The GM COULD have changed that for her, no one but him knew what the cloak was, but he'd already planned it and squashed her just as much as he would have done us. Yeah, he took more jokes about sleeping on the couch for it, but the rules remained fair.