I’m not expert and we do need to consider that Hook and Milah live in a world that is not regulated like ours, but could they have a common law marriage? They were obviously deeply in love, they lived together for 5-10 years like Keb pointed out, and once Hook had Bae (even for a short time) they “shared” a child.
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"He was a lot of things to me" "The only conclusion was love"