I never heard that theory. I remember there being one that he was in a relationship with the Raven Queen though.
Perhaps it was just in the circles I ran. It came from the "Kord is couch-surfing in Celestia" idea, which is kinda-sorta true, in that he betrayed his mother (Khala, former goddess of winter) during the War of Winter to join up with the deities opposing her. Kord switched sides because he saw, after a pitched battle against Moradin, that his reckless blows were hurting innocent mortal bystanders, and he realized he wasn't really
about that. Being "homeless," he was offered a place to stay in Celestia...which is a bit weird when you consider he's more or less Chaotic Good, with the emphasis on the
Chaotic, yet apparently happy to live in a realm defined by the two primary LG deities. Somewhere along the line that produced the idea that it's more than just alliance, but affection, that keeps Kord there rather than going off to find a place more his speed (such as Arvandor, which also has only two deities) or just not have any home at all (like Melora and Avandra, who don't have any divine domain.)
Plus, it's a neat subversion of the usual storm god/giant serpent dichotomy found in Indo-European mythology (and sometimes outside it.) That is, usually the storm god (Zeus, Susanoo, Thor, etc.) must fight and defeat (or sometimes be defeated by) the giant serpent, with subversions being relatively rare (such as the Epic of Mwindo, where Nkuba, the storm god, is
blood brothers with Kirimu the multi-headed giant serpent.) So it's kind of neat, IMO, to have something where the storm god and
a great serpent are in a positive rather than antagonistic relationship. (And it's not like D&D pantheons are exactly bursting with positive, wholesome male couples, y'know?)