A college student described today the long and agonizing process he went through before deciding to defy the Selective Service registration law, but the court would not let him explain his motives for refusing to register.

Benjamin H. Sasway, 21, the first man indicted for refusing to register under the new law, told how he littered his home with drafts of a letter to President Carter and engaged in repeated, heated discussions with his parents.

However, U.S. District Court Judge Gordon Thompson Jr. ruled that, under the rules of evidence, Sasway's reasons for defying the registration law were irrelevant, so he was repeatedly denied a chance to explain to the jury his reasons for risking a five-year prison term and a $10,000 fine.

Government prosecutors and the federal judge also refused to allow Sasway's attorney to tell the jurors how severe a penalty Sasway might suffer if they found him guilty of not signing a registration card.

"I think that we have witnessed a very good case of the law acting inhumanely," said Sasway, a political science major at Humboldt State University in Arcata, Calif.

The continual objections to testimony about Sasway's motives, regularly sustained by Judge Thompson, suggest that anti-war activists will find it difficult to dramatize their wide-ranging objections to the registration system at any of the other approximately 70 cases against resisters now being prepared for trial.

"I didn't get to introduce three-quarters of what I would have liked to," said Charles T. Bumer, Sasway's attorney, who rested his case late yesterday. Asked about the limits on testimony, U.S. Attorney Robert Rose noted that most of the anti-draft cases during the Vietnam War also were adjudicated quickly because legal precedents made motives irrelevant.

If Sasway is convicted, Bumer has said he plans to appeal on several grounds, including the theory that the framers of the Constitution gave only the states, not the federal government, the right to force involuntary conscription.

Bumer also plans to challenge Sasway's indictment on the grounds that the government discriminated illegally by targeting a few outspoken registration resisters in parts of the country where popular sentiment would be against them, such as this retirement community for military officers.

So far, five men have been indicted for not registering and one, Enten Eller, was sentenced to three years probation and ordered to register within 90 days by a judge in Roanoke. Selective Service officials say that, as of July 18, 74,000 of nine million eligible American men had not registered, but that so far only those who went out of their way to identify themselves have been indicted.

In his testimony today, Sasway said he began thinking about resisting registration in the spring of 1980 after President Carter announced plans for a registration system without a draft to show American displeasure at the Soviet invasion of Afghanistan. When Sasway returned to his home in Vista, Calif., 35 miles north of here, for summer vacation, he discussed his plans with his father, Joseph, 49, and his mother, Dolores, 50.

"There were constant discussions about it day and night," Sasway testified. He said he was "very much concerned" about the penalty for not registering, but when Bumer asked him what that penalty was, assistant U.S. Attorney Yesmin Saide Annen objected and the judge told Sasway not to answer.

Dolores Sasway took the stand to say that she typed her son's four-page letter to the president, corrected some of his grammar and tried to make some editorial changes. "I put some 'I thinks' in it which upset him," she said. "I was trying to make it softer." Sasway's parents have both said outside of court, however, that they stand behind their son's decision to violate the law.