As sure as the stars above (i'd really like to kiss you)

"Ashton yawns and checks his phone, only to see it’s already two am. He untangles himself from Luke and stands up. “All right, I’m off to bed. Good night.” He says, leaning forward and pressing a soft kiss to the corner of Luke’s mouth before shuffling out of the room.

It isn’t until he’s actually in front of his bedroom door that he realizes what he has just done.

He just kissed Luke."

or, Ashton accidentally kisses Luke one night and it sort of becomes A Thing after that.