On Michael G. Simoneau's 16th birthday he and some friends drove to the Iwo Jima memorial statue in Arlington, a known homosexual meeting place, to "find a homosexual and rough him up, humiliate him . . ." Simoneau's own lawyer said yesterday in court.

Simoneau and his friends picked at random Ronald J. Pettine, a former aide to Rep. Morris K. Udall (D-Ariz.) and a married man with two children, who was engaging in homosexual acts with two other men at the memorial, according to testimony in the opening day of the trial in Arlington Circuit Court.

Several hours later Pettine's nude, battered body was found by police. He had been beaten to death with a tree limb. Now 17, Simoneau, is charged with armed robbery and murder in connection with the incident.

The prosecution and defense, in opening statements, outlined their versions of the incident, which occurred at the memorial in the rain-soaked early morning hours of Oct. 2, 1976. John A. Keats, Simoneau's lawyer, said he would concede most of the prosecution's evidence, but argued that his client should be found guilty of manslaughter, not murder.

Testimony included that of two men who said they were engaging in sexual acts with Pettine shortly before he was killed. Pettine's wife, Elizabeth, also testified about the last time she saw her husband alive and said she knew at least three years before his death that he was a homosexual.

Somoneau, a Fall s Church juvenile who was certified to stand trial as an adult last March, is accused of robbing Pettine of his clothing, cash and a gold Dunhill lighter. He is also charged with murder during the commission of an armed robbery, a charge that carries a maximum penalty of death. Assistant Commonwealth's Attorney Kenneth Melson said yesterday he would not seek the death penalty against Simoneau, but declined to comment on his reasoning outside the courtroom.

Simoneau is the first of three men indicted on murder charges to stand trial in the case. Charles A. Bamman, 21, of Arlington, and Alan J. Arnone, 22, of Maryland, are scheduled to stand trial later this year.

Elizabeth Pettine, the first prosecution witness called yesterday, testified that on Oct. 1, 1976, several hours before her husband's death, she and Pettine had attended a show at the Kennedy Center. They arrived home at about 11:30 p.m.

"He was feeling quite tense that evening," Mrs. Pettine said. "He said he felt like going out for a drink."

She said she watched her husband put on a blue jeans, a denim jacket and boots. Mrs Pettine, a teacher, said she earlier had given her husband, who at that time was unemployed, $25.

Mrs. Pettine said she was aware that her husband had been a homosexual for "about three years," and that he mixed publicly with homosexuals.

Michael Sumerlin, an architect, testified that he and his roommate went to the memorial at about 1:30 a.m. on Oct. 2 "to find a sexual partner." Sumerlin said he walked around the memorial's wooded area for several minutes, saw Pettine, then followed him and another man, identified in court as Robert W. Heuer, into the woods where they engaged in sexual activity.

During that time two men entered the woods and Pettine soon went toward them, Sumerlin said.

"The other guy and I were standing there just a few seconds before I noticed the two guys beating the victim," Sumerlin testified. "The two people and the victim were all kind of tangled. The two people were standing over the other guy, the victim."

Sumerlin said he than ran from the woods and heard Pettine scream, "Help me, help me, they want to kill me."

Sumerlin said he then ran from the Pettine before theincident. When asked by Keats if Simoneau was the assailant, Sumerlin said he did not know, Heuer testified that he was engaged in sexual activity with Pettine and Sumerlin and that he ran when he heard "a very loud thumping noise and I saw a man on the ground with two men above him."