So, I was curing zombies to restock my zombie-slaughtered village... the first two got ordinary male villagers, but then the third produced a married couple in the cage, while two guards spawned outside the cage! The guards were one of each gender, but happily not married.

That's pretty interesting that you got two villagers from one zombie! I wonder if they get sort of randomly created like your family members do when you begin... you know, when you start with a spouse and 0-3 kids?
As for the guards, they will generate per number of villagers. If you look in the config file, you can set how many villagers there have to be before a guard will spawn. I found I had too many guards hanging about so I turned the number up a bit. That's probably what happened.

Yup, probably due to naturallySpawnVillagers spawning a spouse. This will also happen in 1.9.