okay, let's keep it real, here. if the person you're involved with is married, you knew that going in. if the two of you have a relationship...and have boundaries within that relationship...and you break those rules/go beyond those boundaries, without letting that person know...you're cheating. yes, you have a right to find someone. but, so does the married person.
the first thing you would tell anyone is to get out of the relationship you're in before moving on. it doesn't matter if the person you're involved with is married or not, you're in a relationship with that person. GET OUT OF THE RELATIONSHIP FIRST...THEN MOVE ON. Otherwise, you're cheating.