I am a firm believer in there being more to the story as to why someone might seek a relationship discreetly outside their primary relationship. I wouldn't cheat on my partner. I have a very communicative and understanding relationship with my spouse. Unfortunately not everyone does. I don't feel compelled to force them to have an open relationship in order to have a relationship with me. It's not always possible.
Because I don't take issue with a potential partner being spoken for and me being a side show rather than the main show...it doesn't say that I have no boundaries or morals. It doesn't mean that person is all over town having sex with everyone else. This married person could be having sex with only one person. Some people haven't had sex in years in their own marriage for whatever reasons and truly want to fall in love with someone on the side in order to have a reason to feel like living again. Not all married people who have affairs are liars in other areas no more than someone who is lying in other areas is going to be prone to have affairs. One does not have a direct correlation with the other in my book or my experience.
Those who can't tell their spouse are not necessarily inherently dishonest people in every area of their lives is what I'm trying to say. Who is to say if you were their primary spouse they would even need a discreet relationship like this?
It doesn't mean they would do the same thing to you if given the opportunity. Chances are you are a different persona entirely from their primary partner. Which is the reason they feel comfortable sharing with you that they are married but not sharing with their wife/husband they are having a relationship with you.
I consider myself to be a "confidante with benefits" at that point. It is something so much more than friends with benefits but not quite the type of partner you can move in to your home if that is what you are looking for. It's a limited relationship for obvious reasons. Limited because of what they can't share with the other person. Not limited because they are bad people. They might have a lot of integrity to keep their family together. I can respect that. You just have to know what you want and what limitations are OK with you.
I hope this helps.
Me: 32, bi, female, married 14 years to R.
R: 33, hetero male who is polyfriendly, NSA friendly under the right circumstances.
S: The child R and I have together.
Neither have found the partner(s) we would like to merge villages with. In the meantime we are exploring our collective sexuality to find a frequency that feels right.