The Harbinger of the Companions is highly regarded through Skyrim as wise, thoughtful and skilled in whatever they choose to do. Since being named Harbinger by Kodlak, the Dragonborn's lived up to the image. On the other hand, everybody knows Farkas isn't the brainy one of the twins, so there are bits and pieces of gossip floating around Whiterun when the Dragonborn decides to marry him. Not much, mind you, but enough to make Farkas start wondering why someone like the Harbinger would pick him at all.
I want an awkward confrontation about that very wonder, the Harbinger explaining exactly what drew them to him, and some sweet, emotional, good old comfort sex. Anything else is fair game, including race and gender of the Dragonborn and who Farkas hears those lovely rumors from. Have fun!