SAN FRANCISCO--Violet the orangutan is on the pill. King L'ear the bison was rendered harmless with a bovine vasectomy And Maxine the cougar is using a Norplant-style implant.

Birds do it. Bees do it. But at the San Francisco Zoo, some of the romantically inclined are taking precautions.

These days, zoos hard pressed for space have adopted strict breeding practices to prevent unwanted offspring ending up in dismal wayside attractions or being used as quarry at private big-game ranches.

"It's nice to have little bodies, it's a crowd pleaser and things like that, but we've got an overall responsibility," said zoo veterinarian Dr. Freeland Dunker.

In honor of Valentine's Day last week, the zoo ran a special R-rated sex tour jampacked with fascinating facts. Among others:

Foreplay can last from over a month for rhinos to hours for the aptly named slow loris, a tree-dwelling primate.

Orangutans can do it upside down, while rhinos carry on for up to an hour. Lions take pride in engaging up to 50 times in a 24-hour span and koalas mate for just 40 seconds three times per year.

Then there are the zoo's two female Canada geese, who have laid more than 40 unfertilized eggs for each other and are inseparable. Named Gertie and Alice--after Gertrude Stein and Alice B. Toklas--the pair are favorites of animal keeper Jane Tollini.

"They show no interest in being apart," she said. "If one is gone, the other one stands out here just screaming."

As guide for the no-children Valentine tour, Tollini takes a keen interest in the sex lives of her charges.

This year, the tour includes information on contraception, such as King Lear's epididymectomy. The procedure, which involves cutting the sperm channel, eliminated unwanted baby bisons, but didn't stop King Lear from being dominant bull.

On a recent tour, Tollini pointed out some of the contraceptive stars, such as Violet, slouched in the entrance to her cave munching on leaves. Since 1986, Violet has been taking human birth control pills, straight from the supermarket.

Maxine has a small cylinder of rubber inserted between her shoulder blades--the implant works on the same principle as those used in humans, diffusing a conception-inhibiting substance. THE QUOTE OF THE WEEK

"If the condom isn't effective, the University doesn't want to be liable for it."

--Sylvia J. Struss, a Harvard trademark administrator, explaining why her office turned down an offer to put Veritas on contraceptives.