I would be interested in what others on here think about that as well, vis a vis the purposes and goals of WikiTree.
I think the answer to that depends on the circumstances. It is possible that the parties were married under the laws of the state and the record, for a variety of reasons, doesn't exist anymore (the courthouse burned) or hasn't been located (those pesky relatives who were on the move when they got married). Also, I'm no expert on common law marriage, but it was a valid, legal marriage even though it wasn't performed in front of minister or justice of the peace. Then there will also be those cases where parties lived as man and wife and were not permitted legally to get married, such as under miscenegation laws or because one of the parties was unable to dissolve a prior marriage.
I would suggest that if "spouse" most accurately explains the family relationship, it would be okay to add them as that, but I would also explain, explain, explain in the biography section. As an example, although this profile is private, I have a living relative who has been in a long term (30+ year) relationship with a person who is, within the extended family, treated as her husband. They have no children. I am not sure how else but "spouse" they could be connected in WikiTree. I also have a relative, who died in the 1960s, who was in a long term relationship with a person of the same sex. I have chosen not to include that person's partner as a spouse because they chose not to acknowledge their relationship publicly during their lifetimes.
And because of how sensitive this could be, I would also suggest if it involves people in the recent past who may still have close living relatives, to tread very carefully.
What do others think?