TV is slapping a chastity-belt onto primetime.

The big broadcast networks are showing less sex on TV these days, says a prominent TV watchdog group.

ABC, CBS, NBC and Fox are all showing less sexually charged content during the so-called family hour -- 8-9 p.m. -- but risqué is on the rise on the smaller nets, WB and UPN, according to a new study from the Parents Television Council.

The study, called the PTC's "State of the Industry" report, compares sexual content, defined as nudity, sexual innuendo, suggestive comments or jokes and references to specific sexual acts, in 1998, 2000 and 2002.

It found that overall, sexual content is down by 9 percent across all the broadcast networks during the first hour of prime time and down 12 percent in the second hour since 1998.

CBS execs say there is no explanation for the dip in sexual content on the network.

"We've always felt that, at 8 p.m., we were squeaky clean," CBS chief Les Moonves said yesterday.

"I can tell you that this was a surprise," said L. Brent Bozell III, council president of the study's results. "I think there's a market demand for [televised] garbage that seems to have run its course.

"I think that sponsors are playing a role in this in that they're getting behind positive," he said. The council pushes for less sex and violence on television.

ABC showed the greatest decrease in sexual content during the family hour, with a 67 percent dip, followed by a 48 percent drop at Fox.

Sexual content on UPN was down 12 percent, and 6 percent on CBS. WB was up 88 percent. NBC went up roughly 45 percent between 1998 and 2002, but showed a more recent decrease of 34 percent between 2000 and 2002.

During the second hour of prime-time, sexual content on Fox was down 80 percent, followed by ABC, down 72 percent, and CBS, down 39 percent. Over the four years between 1998 and 2002, sexual content on NBC went up 32 percent, but has dipped 37 percent within the last two years.

UPN had an increase of 20 percent and WB 310 percent -- the increase was huge, but the numbers used to make the comparison were so small due to its limited schedule.