Action, reaction, motivation, emotion, all have to come from the characters. Writing a love scene requires the same elements from the writer as any other.
Nora RobertsRoarke: The bodies of the three men were found floating in the Chattahoochee River. Eve: I think it'd be embarrassing to be dead in the Hoochie-Coochie River. Roarke: Chattahoochee Eve: What's the difference? Roarke: Quite a bit, I'd think.
Nora Roberts