Despite common prejudices and some scientists' past beliefs, homosexuals are no different in their physical or biological responses to sex than anyone else and deserve to be treated as normal human beings, the nation's two leading sex therapists, Dr. William H. Masters and Virginia Johnson, maintained today.

They said a 15-year study of more than 300 homosexual men and women has convinced them that homosexuality for most persons is neither a physical or emotional illness nor a genetic disorder, but rather a form of "learned behavior," just like heterosexuality, or preference for the opposite sex.

But homosexuality is also a form of behavior that can be unlearned in a surprising two out of three individuals, Masters and Johnson said, if the homosexual wants to do so and gets their kind of treatment.

The famous husband-and-wife therapist team urged doctors to help sexually ill-adjusted homosexuals who want help.

Like many, heterosexuals, some want help in achieving their own form of sexual fulfillment, Masters said. Others may want to "convert" or "revert" to heterosexual life. In either case, he argued, they deserve objective care rather than the "homophobia" that has made many doctors turn their backs to this large population.

There are an estimated 20 million to 21 million homosexuals in the United States today, and, in the population as a whole, one adult male in three and one woman in five has at some had one or more homosexual experiences, Master said.

Next Monday the St. Louis therapists will publish their latest book-"Homosexuality in Perspective"-as part of their series of books based on their two unique contributions to sexual studies.

They have observed both heterosexual and homosexual couples and individuals in nearly 6,000 sex acts in their St. Louis laboratory. And, working in male-and-female therapist teams, they and their coworkers have given two-week courses of sex therapy to hudreds of mainly heterosexual couples where one partner or the other suffered from some problem as impotence or inability to achieve orgasm.

At a press briefing conducted by their publisher, Little, Brown & Co., the St. Louis therapists reposrted on the result of their laboratory study of the sexual practices of 92 homosexual men and 84 women-all of them functioning successfully and all but 10 of the men living together as "committed couples." They also reported on their treatment of another 151 homosexuals, again mainly couples but including 54 men and 13 women who were dissatisfied with homosexual life and wanted to change.

Laboratory studies were conducted mainly between 1964 and 1968, the treatment from 1968 to 1977. The therapists deferred publication until they felt thay had a fairly complete picture of homosexuality, and could compare the homosexuals with heterosexuals observed or treated in similar ways.

They said today that:

The homosexuals they observed were fully as capable of orgasms as heterosexuals, and women, both homosexual and heterosexual, were fully stimulated.

There were no physical differences in the way homosexuals responded to manual or oral stimulation from the way heterosexuals respond to similar stimulation or intercourse.

The thrapists reported a "20 percent failure rate" in treating heterosexual couples, meaning one couple in five still has sexual problems five years after therapy.

But in treating homosexuals, the therapists have had only a 12 percent failure rate, possibly because the expectations and complications in heterosexual marriage are far greater than in many homosexual relationships.

Also, said Johnson, heterosexual couples commonly tried to achieve sexual satisfaction together, and even simultaneously, while homosexual partners generally "take turns" in satisfying each other, a far simpler goal.

The therapists successfully helped two out of three of the 54 men and 13 women who wanted to change to heterosexual life.

Most psychiatrists and therapists said such conversions are almost impossible in most cases, and at the most have reported less than a 25 percent success rate.

Masters and Johnson worked with individuals who wanted very badly to make the change. But rather than the usual psycotherapy, or "talk therapy" alone, A,sters and Johnson used the same kind of therapy they used with all other "sexually dysfunctioning" persons-a joint course of treatment and practice sessions, conducted in private, for the affected individual and a cooperating partner of the opposite sex.

"We don't urge any homosexual to change," Johnson said. "We aren't attaching values here."

The therapists even found 12 individuals whom they called "ambisexual" persons who are not only bisexual but who don't care whether they have sex at the moment with a man or a woman and who make no emotional attachments.

The St. Louis scientists don't say that society should not attach values. They themselves have often stated their own preference for sex as part of a marital commitment.

"What we are saying," said Johnson, is that homosexuals are all "individuals with human needs and desires," not persons who should be assigned 'to some inhuman role."