Gay, straight, or bi — one of the first assumptions we make about a relationship is that it begins with sexual attraction. Maybe that feels like butterflies to you, or flushed cheeks, or being tongue-tied and awkward. But what if that feeling is entirely foreign to you? What about desire without sex? Angela Chen explores the contradictions — and the possibilities — in her new book, “Ace: What Asexuality Reveals About Desire, Society, and the Meaning of Sex.”