The episode in question - aired on 20 November 2001 - "included a scene in which the characters Buffy and Spike engage in sexual intercourse". The complainants claimed this to be indecent.
The FCC disagreed, and ruled (PDF) that the programme "was not sufficiently explicit or graphic to be indecent. The Commission noted that there was no nudity, and there was no evidence that the activity depicted was dwelled upon, or was used to pander, titillate or shock the audience".
The PTC (motto: "Because our children are watching") has not yet posted its response to the ruling on its website, but leads rather with "Summer's Pop Music Meltdown" by deliciously-named organisation prez L. Brent Bozell III, who asks:
Want a primer on societal meltdown? Then turn on the Top-40 pop station in any town and sample the cultural depths to which too much of today’s popular music has sunk. Summer time is often time for young people to take car trips with the radio blasting, off to the beach, or the movies, or a ball game. How many will fondly look back two or three decades from now and say, "I really loved those rap songs about thugs, drugs, and pimping"?
A good point. And who for that matter will remember "that episode of Buffy where her and Spike got down and dirty"? Answers on a postcard to the PTC, if you please. ®