The Crimson Days Grimoire card talks about two Guardians, a Male Warlock and a Female Hunter, creating a close bond on the battlefield while fighting the Cabal. They don't become a "couple" per say, but it was for the Valentine's day event, it says they became a permanent fireteam afterwords, and that they share their riches and renown together. That's the only straight couple in Destiny, that I know of at least