Sex is not a subject normally reserved for presidential press conferences, but Jimmy Carter took that one in stride, too. And why not? His latest press conference demonstrates anew that at those sessions he's the master.

In yesterday's performance Carter faithfully followed the old Broadway adage. He left 'em laughing. What's more, even applauding. That hasn't happened in Washington since - well, since so long ago no one can remember exactly when.

But if sex was the leavener, surprise was his weapon. He employed it as skillfully as Lyndon Johnson in his most Machiavellian role.

Everyone in Washington knew, it seemed, that Carter was going ahead with the B-1 bomber. The proponents were pleased, the opponents gloomy, the cynics ready to pounce. The I'll-never-lie-to-you I'resident surely was about to renege on one of his most oft-cited campaign promises.

If you picked up your Post in the morning, you learned that Jennings Randolph, the West Virginia senator, had emerged from a breakfast with the I'resident believing Carter would approve production of "much more" bombers than had been thought; that Carter's gray eminence, Charles Kirbo, was passing the word in Atlanta about "adjustments" being necessary between campaign rhetoric and Washington realities; that anonymous White House aides, known to be against the big bomber, were downcast and murmuring about bad news forthcoming.

All that was part of what had become a standard script. Three weeks ago, two liberal Democratic congressmen, Drinan of Massachusetts and Dellums of California, came out of a 40-minute meeting between Carter and B-1 opponents, spreading gloom. "I think the President is leaning very definitely toward building it," Drinan declared. Dellums agreed.

Only two days earlier, Carter conferred with congressional supporters. They came forth with good news for their side. Carter, Barry Goldwater announced, "is becoming convinced we can't live without the B-1."

In the days since, Carter continued the suspense. At his last press conference, he evoked the image of the loneliness of command, and the difficulties of the decision-maker. It was time, he said then, "perhaps on my own and perhaps in a lonely way," to make the final judgment.

Yesterday he began by striking similarly introspective tones. It had been "one of the most difficult decisions that I have made," he said, without saying what it was he had concluded. Then, drawing out the last bit of drama, he gave the news: ". . . I am directing that we discontinue plans for production of this weapons system."

Jimmy Carter went on from there to dominate another press encounter. He was alternately serious and deft, solemn and humorous. And he was always in command. One of the old Washington chestnuts, dug up after every favorable presidential performance, presents the accolade. "He was presidential." Cliche or not, Carter was presidential yesterday.

He was understated and calm about problems with the Russians and the Israelis, diplomatically delicate about saluting both mainland China and Taiwan, statesmanlike on the prospects of the Panama treaty, and casually effective on the new domestic questions asked.

Now on that income tax gesture, and waiting a week to pay $6,000 after knowing he owed no taxes, was that just an afterthought?

No, Carter explained. That was planned. He'd tried to get the IRS to put some $70,000 in income from his book onto his 1976 taxes, but that was disallowed, leaving him still with zero dollars owed. "I consider it to be a problem not because there was anything improper about it," he then said, "but I think that I, as President, ought to demonstrate that the present tax laws are not adequate and that someone who earns as much as I did in '76 ought to pay taxes. That was the reason for the delay."

In short, a noble precedent.

Now about Democratic politics, and the degree of support all good party people ought to give? And what about his own plans on the New York City mayoralty ticket?

Jimmy Carter, the pol, straddled that one nicely. "My general belief is that Democrats ought to support the Democratic nominees." But: "every Democrat, every American, can reserve the right to participate with varying degrees of commitment or intensity or enthusiasm."

In short, another restatement of the classic all-things-to-all-people political doctrine.

Now about making a decision on the future of the U.S. Postal Service, (and everyone knows how well that works)?

No decision yes: then the sly aside: as the press certainly knows the Postal Service's governing board is completely independent of the President, and "I have no responsibility for the Post Office."

In short, don't blame me.

It was left to Lester Kinsolving, the Episcopal priest turned press corps gadfly, to raise the question of sex. Kinsolving, in a variation of the when-have-you-stopped-beating-your-wife question, thundered dramatically that even though Carter's relationship is monogamous, he didn't seem to hold anything against people working for him who were otherwise.

Right about himself, Carter responded, to laughter. What about the other part, and those people "who were promiscuously with other women?" Kinsolving demanded.

Softly and with a smile, Carter answered:

"If there are some who have slipped from grace, then I can only say that I will do the best I can to forgive them and pray for them."

In short, a soft answer still turneth away wrath. And still wins points for a President.