From the season one* episode of Justice League Unlimited "This Little Piggy".
In the episode, a woman named Circe magically transforms Wonder Woman into a pig and she only agrees to change her back if Batman sings. Great Bat-moment or greatest Bat-moment? The writers really cockteased fans with a possible Batman/Wonder Woman relationship, but despite it being obvious that they were into each other, it never came to be. I'm assuming DC was responsible for that. Bastards.
*Really, it was season three, since JLU was just a continuation of the Justice League cartoon with an expanded roster of characters.