Zombies definitely couldn't climb fences
I might be misremembering, but I didn't think the zombie actually got over the fence but sort of made a half-baked attempt to get over the gate, failed, and then the walkers knocked the fence down.
They certainly didn't successfully climb the ladder early in the episode either so I don't get the feeling that the zombies will be able to overcome a fence that isn't easily pulled down by the weight of the crowd.
Oh they definitely climbed the gate before the weight of all the other Zombies knocked it down.
They showed at least three get up to the top and then Rick shot them down, then while they showed Glenn getting the keys to the truck (amazing lucky IMHO to find and then get the right truck) they show Rick shooting several more times. Then as they get in the truck one final Zombie jumps the gate and trys to get Glenn just as he gets in the truck. At this point the gate falls.
Maybe if a Zombie see you do it the Zombie can do it. Or attempt to do it.
Zombie see, Zombie do?
Makes some sense.
As for the "affair" between the wife and friend........I really hope that is dealt with and over in an episode as I could sincerely less about a love triangle in the context of an apocalyptic zombie story. I didn't care for them in Lost and I certainly don't think this series will benefit from their use beyond just being an easy crutch for writers to lean on for easy story-telling.
You ain't going to have to worry about that.