Sex is sex is sex ... unless it's sex between the undead.

Buffy (Sarah Michelle Gellar) and Spike (James Marsters) did the nasty zombie during Tuesday night's episode of "Buffy the Vampire Slayer" on UPN, and while the content would make even adults hold their hands over their eyes ... things originally were much steamier with between 10 and 15 seconds being cut from the scene.

"You should have seen the shot at the end before UPN cautioned us," co-executive producer Marti Noxon told Zap2it. "It was much, much longer and that last image of them in the smoke -- that's CGI smoke we had to add for a couple reasons. One of the reasons is that we had to shorten that shot dramatically, but we also needed to obscure the action for the network."

However ... "I think it's the show against the wall that's really the humdinger, and they didn't object to that," Noxon said.

There were several discussions going on about the scene among the writers of the show, especially the sound of a zipper after Buffy's hand disappears below the frame toward Spike's waist.

The sex scene, while a definite step forward between Buffy and Spike, isn't the end of the journey the two will continue to take, Noxon said. But it's a relationship that won't be long-term, either. And, anything to get Spike's clothes off is definitely an option, she said.

"There's actually a lot more flesh," Noxon said. "From now on, actually, we keep finding reasons to naked [Spike] up."