We were given the same guidance re: naming only one person. We have my step brother named. The idea is that the kids would go to him and his wife if something happened to us. However, if they divorced prior to/during/after the guardianship, it is our wish that the kids stay with him. We also don't want a custody battle between them for our kids. It's terrible to think about, but there is a lot of life insurance money if we both pass at the same time. There are people who would fight for custody of the kids just for the purposes of getting the money.
My understanding of the successor is that it goes for if the first person is unable or unwilling. So if we both pass, and my step brother is unable or unwilling to take the kids for whatever reason, they would go to the successor guardian. I believe once he takes guardianship, he would need to make his own provisions for what would happen to the kids if something then happened to him.
I've never heard of any type of preference towards married people. It's your decision who you want to take your kids if something happens to you. If you prefer a married couple, I guess that would be your own decision to make. We chose based on who we thought would have the time/energy/desire to raise the kids, as well as whose parenting style we admire.