“Pillion,” the word for the back seat on a motorbike, becomes an apt metaphor for a romance structured around riding behind someone else’s desire, will, and direction. The film Pillion, written and directed by Harry Lighton, is a gay-drama-romance structured around an asymmetrical relationship whose very premise is power: Colin (Harry Melling), a sensitive holiday carol singer and self-effacing “wallflower,” becomes fixated on Ray (Alexander Skarsgård), the strikingly attractive leader of a motorbike club.