Why does some women prefer married men rather than single men?

Competetion. It's an inherent behavior in many people. It's more fun to some people if they are doing something taboo as well. Gives them excitement that they may not otherwise be able to fulfill. I wouldn't characterize all or even a high percentage of women as having this personality type, but some do. Unfortunately that means that some marriages are broken up...good or bad in the long run, it's just part of the human spectrum of behaviors that make this world what it is.

Also, some people are just jerks. But what goes around comes around.