I am polyamorous and believe in the ability to love more than one and be committed to more than one. This can cover a wide spectrum. link
I think no matter if you are married, in an open relationship or single and dating, honesty and ethics always need to be present. Communication is key.
Yes, if one person in a relationship wishes to date outside the relationship, that option should be available for the other person though they may be perfectly happy where they are at.
Legal marriage is likely not in the cards for me, that is my own choice. I have forged my own path, I have children and have a successful relationship with their father (now an ex) and his wife and their two kids. I currently have a BDSM Daddy/girl relationship and play partners. There are more than a few people I am interested in knowing better and likely dating, but time conspires against us for now.
My life is never dull. The cards are always on the table and I am generally very happy with the way things are going.