There arent any 'rules' about PW as i see it. People will use the technique as they see fit, and according to their own moral compass.
How do you define married? Some will say you make a public declaration of your union, others sign papers, others will say you are only married after a union is 'consummated' - does this mean any two people who have sex are married? What about defacto partnerships, some people live together for years without a ceremony - are they truly married or fair game just like boyfriend/girlfriend relationship? What about a defacto relationship with children?
Whether or not there are consequences for PWing a married person is different for every situation, i personally dont believe that every couple in said situation has issues that are much different to any other couple. You get together, however it happened and you work on your relationship.