There's no sex scene. It's alluded to and there's some innuendo -- part of it was seen in a trailer or clip of Harley telling Nightwing to be good and he might get a cookie. Long description if you want. I suppose I don't have to use spoiler tags but whatever: In my opinion, it was a part of farcical nature of the movie. The Superbabes serves to show how women in the genre are objectified then Harley turns the tables on Nightwing and objectifies him, then figuratively throws him away during the Batmobile drive. It never went as far as the Deadshot/Harley scene in Assault on Arkham for example.