Jesuit Father Francis L. Filas came to St. Michaels School in Annandale yesterday to spread the word of the "goodness of sexuality" to an assembled group of seventh and eighth-graders and to warn them of the consequences of discovering the joy of sex before signing a marriage license.

Father Filas came to the Roman Catholic Diocese of Arlington from Chicago as a "kickoff speaker" to test parental response to a possible sex education program for Catholic students in grades. 7 through 12, according to a diocesan spokesman.

A professor of theology at Loyola University in Chicago, Father Filas is on a 10-day Virginia tour, speaking to about 18 groups of Catholic school children and parents on the subject of sexuality.

"We live in a society that rips you apart," Father Filas told approximately 200 young girls sitting quietly in their crisp white blouses and blue and plaid jumpers. "So many girls can be played for a sucker." They were having this talk, Father Filas told them, "to prevent you from getting hurt."

They also were having the talk, said Doug Grasso, diocesan coordinator of CCD and Adult Education, "in response to the controversy" that has surrounded the subject of sex education in Fairfax County.

Last month, the Fairfax County school board approved in county schools'. The instruction will not be coeducational and will not include information on abortion, birth control, homosexuality or masturbation.

Father Filas, however, managed to hit every one of the proscribed subjects, illustrating his points with gothic horror stories of misused sexuality that he delivered with Gatling gun speed. "We want to get you before the culture gets you and tells you sex is for fun, you can play around with it and so what," Father Filas said. "I'm here to show you the positive siad."

The message, Father Filas, said, was simple. First, "sex is not what we do but who we are," and second, "Always use refined and delicate language and as soon as possible use technical termonology."

Before launching into a discussion of the perils of premarital sex, Father Filas stressed the importance of communication and serve up a number of anecdotes depicting what can happen when ignorance is something less than bliss.

"Girls should understand prementstrual tension," Father Filas said. He recounted the story of a woman with husband and child who, seemingly unaccountably, he said, began to have the desire "to cut her baby's throat and strangle her husband." For six months, Father Filas said, she lived with these strange desires until she went to see a doctor who promptly recommended psychiatric hospitalization.

After years of hospitalization, Father Filas said, "she was released as a hopeless suicide." Eventually, she met Father Filas, who said he suggested that the problem might simply be premenstrual tension. Ever since, Father Filas said, she has been a "happy wife and mother."

After a discussion of male prostitutes ("scruggy vermin") wet dreams ("God's way of telling a boy he can father children") and masturbation ("is that true happiness? . . . Where's the meaning?"), Father Filas talked about tempermental sexuality, or the "biocultural differences between men and women."

"If you don't hold with the differences between men and women," Father Filas said, "then you destroy marriage."

There followed a catologue of male-female difference that would probably not have a positive effect on Gloria Steinem's serentiy. "Men," said Father Filas, think in terms of persons . . ." In addition, said Father Filas, men and women have different senses of humor. "Woman is the diplomat."

After complimenting his audience on the fact they hadn't "snickered" once furing his lecture, Father Filas asked the girls to write down anonymously any questions they might have about sex. Only one scrap of paper appeared.

"What if a boy makes fun of your breasts because it's hard to ignore him if he keeps on? asked the questioner. "Treat him with scorn and contempt," came the answer. "It's the one thing men cant stand. Say something like 'grow up, buster,' and then turn on your heel and walk away."

Since there were few questions from the audience, Father Filas answered some of those that he said had been asked most frequently in other lectures. One of the most popular, he said, was whether a girl should have premartal sex or "who not try it before you buy it?"

"When people say sex play around with sex, they forget about what can happen," Father Filas said. "He's using you, he'll drop you."

In addition, Father Filas said, there were the dangers of pregnancy and "social diseases." Syphilis, he noted, can eventually cause insanity and might have affected the behavior of such past and present leaders as Ugandan President Idi Amin and Ivan the Terrible.

Father Filas closed his lecture with these words: "You are good, you are beautiful, you deserve love."

"I think he's stuck up," said Phyllis Lepri of Fairfax City, who, with her daughter Dana, was in the audience. "He didn't get down to the nittygritty, like what does a girl do when she's sitting in a car with some guy and he's got his arm around her shoulder and reaches for her breast?"