For me it's the Merlin character's behavior on BBC's Merlin -- the extent to which he ignores all the Pendragon genocidal habits and sacrifices himself & others for the sake of Arthur just does not make sense to me if you assume Merlin is totally heterosexual with no unrequited romantic pining. I have to believe Merlin is in love with Arthur to make the characterization hold together.

I've had this conversation with my dad a few times (who watches the show on BBC America) and he generally ends up flailing all, "he's not gay! I don't... I don't know, he just DOES IT FOR ARTHUR."
Me: "okay, but how do you explain it?"
Father: "HE JUST KNOWS HE HAS TO."
Me: "Why? What has Arthur really done to deserve that depth of fixation and loyalty?"
Father: "I... don't know. IT'S JUST A MYSTERY. MAYBE HE HAS TO, IT'S MAGIC."
Me: LOL