Vasectomies, tube tying and mass sterilization aren't normal fodder for chitchat at Washington cocktail parties. But when you throw a reception in the name of population control, well these things do happen.

So nobody thought it was gauche or declasse to sip wine in philanthropist Stewart Mott's living room last night while talking about Fallopian tubes and the silicone bands used to tie them.

And Mott, the General Motors heir who threw the party to celebrate the first birthday of the Population Action Council he's chairman of, had plenty to say about vasectomies himself.

First of all, he hasn't had one - but he promises he will.

"I'm a bachelor, age 41, no children, and I was National Non-Father of the Year in 1974," he said proudly. "But at some time I will have children, get married, then have a vasectomy - in that order.

"You know," he added, "women go through menopause in their 40s. I think men, enlightened men, ought to have voluntary vasectomies at age 50 to put them on a par with women."

Many of the 100 or so party guests were young Hill staffers and Agency for International Development (AID) people who were quick to point out they had no children and were never going to have any (at least not more than one or two, anyway). A childless household became the status symbol of the evening.

So Jack Sullivan, assistant administrator of AID for Asia and father of two, had to face a little ribbing.

"I assume both of your sons have had vasectomies," said Herb Stone, administrative aide to Rep. Joe Pritchard (R-Wash.). "That's the only way you could get in the room here."

There were a few, however, who decided a cocktail party was as good a place as any to talk seriously about a subject - like overpopulation - that's no longer chic.

"The problem with the Congress," said one Hill staffer, "is that they don't have the time to focus on something like this.It's certainly not politically expedient in terms of getting reelected."

"The United States is really one of the few countries without a population policy," said a member of the Population Action Council governing committee. The United States, she said, has an annual 0.89 percent population growth rate and is still years away from zero growth.

As for everybody else, the talk ran the gamut from cocktail meatballs to the heat. About half the party was outdoors in Mott's garden, and sweat-stained shirts were everywhere.

But Mott was brave. "I'm as hot as you are," he told his guests."But in the interest of sex and population, I'm not going to take off my clothes." CAPTION: Picture, From left, Richard Benedick, State Dept. population affairs coordinator; Stewart Mott, and Population Action Council director Werner Fornos; by Joe Heiberger