SAN JOSE, Calif. — The 1990s are likely to be the decade when heterosexual Americans take a gamble with AIDS.

A few say they worry. Others say they are careful. But the most straightforward evidence suggests most heterosexuals are pursuing that most universal passion — sex — without any regard to AIDS.

Many simply believe that AIDS is not something that will happen to them.

“You have better odds of winning a lottery than of going to a bar, going home with somebody and getting AIDS,” scoffs Jeff, a bartender at the Gordon Biersch Brewery Restaurant in downtown San Jose, Calif.

Is he right?

Probably not. It’s true that the odds of getting an AIDS infection from a single sexual indiscretion with a stranger are slim. But they’re a lot better than the 1-in-14-million chance of winning the jackpot in 6-49 Lotto, the California lottery.

The odds for AIDS are probably closer to something like one in 100,000. That’s if researchers are right when they estimate that:

An uninfected person has a 1-in-500 chance of getting an AIDS virus infection from a single heterosexual act with an infected partner, and

About four people per thousand are infected with the AIDS virus, overall, in the general population.

So the odds of heterosexual transmission of AIDS in middle America are slim for now. Only 5 percent of U.S. AIDS cases reported so far have come from infections transmitted by heterosexual sex. That may explain why many people — perhaps not aware that, worldwide, 60 percent of the AIDS cases are caused by heterosexual infection — believe they don’t have to worry about curbing their sexual lifestyles.

But somebody wins the lottery. And somebody — heterosexuals included — is going to lose the gamble with AIDS.

Some confess to worrying about the risk of AIDS. But the most indisputable evidence suggests that — even in this era of condom advertising — when the lights are low and the mood is right, most heterosexuals don’t do anything to prevent it.

The strongest proof comes from the fact that rates for other sexually transmitted diseases — gonorrhea, chlamydia and herpes — show no sign of declining in heterosexuals.

Chlamydia infects an estimated 4 million people a year, gonorrhea infects more than 700,000 and syphilis has shown a steep increase — from 70,000 to 110,000 between 1984 and 1989.

“We don’t have any good evidence that … there have been any major changes in behavior,” says Dr. Willard Cates Jr., chief of sexually transmitted diseases at the Centers for Disease Control.

Educating people isn’t necessarily enough. “They are aware of how you get AIDS. They still think `it’s not going to happen to me,”‘ says Naz Motayar, a student health educator at San Jose State University. At least a half-dozen students are infected on that campus, says the campus health director, Dr. Richard Sanchez.

Nationwide, surveys suggest about two of every 1,000 college students are infected. More than three of every 1,000 military recruits are infected. Rates are probably higher in some cities such as San Francisco, where more than half the gay men are believed to be infected and about 12 percent of the IV drug abusers.

AIDS can be spread by sex or by blood entering a break in the skin. Because most people infected so far in the United States are homosexual men or IV drug abusers, many middle Americans think they’re safe if they avoid both groups.

Others say that relying on sexual labels is unwise. “I think any population that’s sexually active is vulnerable,” Sanchez says.

If surveys are right, most Americans are pretty active sexually.

A pilot study done last fall in Dallas County (Texas), for example, shows that 40 percent of men and 21 percent of women have had more than five sex partners in the last 10 years. A smaller, but significant, group had more than five sex partners in the last year: 6 percent of men and 2 percent of women.

Those surveyed were chosen in a scientific manner designed to make the results representative of the general population in Dallas County. Overall, 16 percent of the people surveyed had sex with a partner considered to be “high-risk” for being infected with the AIDS virus.

What does that tell us about America’s sexual habits?

“The sweeping statement, `Just say no,’ has not permeated our society,” says Cates.

Not only are people continuing their sexual pursuits, they’re usually not using condoms. Teen-agers surveyed say they are using condoms more often than they used to, but the reported increase is only from one-eighth to one-quarter of sexual encounters, Cates says.