Donna and Ricardo Thornton are celebrating their second wedding anniversary and are expecting their first child. In some states, they would not have the right to do either.

The Thorntons are mentally retarded. Their rights are restricted in many states, experts say, and even where marriage is allowed, some states take custody of all children from the marriage.

Today, as more and more mentally retarded people are placed in group homes instead of large institutions, more of them are marrying and there is growing debate over whether people with unusually low IQs can be good parents.

"Mentally retarded people are just like anyone else," Ricardo Thornton, 27, said yesterday while seated in the living room of the couple's Northwest Washington apartment.

"I would like to tell people they are no better than we are," he said. "We're trying to make it just like them. Maybe we step a little slower on the way to our goals. But maybe we'll be even greater than them one day."

Opponents of marriage between mentally retarded persons contend that the children of such couples may not learn to talk properly or may have difficulty in school because they don't receive adequate stimulation at home. Proponents say that society doesn't decide who should be parents based on their race or income, so IQ should not be a factor either.

District law doesn't address the issue of whether mentally retarded persons may marry, according to Tony Records, a spokesman for the District of Columbia Association for Retarded Citizens.

"The District treats marriage as a contract between two competent people," Records said. "Once you reach age 18, you're assumed to be competent unless someone raises the issue and you're judged by a court to be incompetent."

Yesterday, Ricardo and Donna Thornton seemed a world away from such controversy. Their baby is due in December. If the baby is a boy they'll name him Ricardo Thornton Jr. and call him "Little Ricky." If the baby is a girl, she will be named either Dana Lee Thornton (her choice) or Evelyn Lee Thornton (his choice).

"I was so excited," said Donna Thornton, 34, recalling how she felt when the doctor told her she was pregnant. "I couldn't believe it. I was so happy. I still am.

"I go in stores now and go look at baby clothes, baby things, little rattles and stuff," she said.

The Thorntons have been classified as mildly retarded, with IQs just under 70, according to a D.C. Department of Human Services official. An IQ of 100 is considered normal.

There is no reason to believe that the Thorntons will have a mentally retarded child, said Shirley Rees, a community liaison for the Bureau of Community Services, a division of the human services department. "Statistically, only between 10 to 15 percent of retardation is genetic," she said.

The Thorntons are still wards of the District government, but later this summer they will be discharged, said Rees. The social services they receive will continue, she explained, but they will be under no legal obligation to participate in the programs. The couple also will continue to live in an apartment building supervised by a nonprofit group that assists mentally retarded people.

Donna and Ricardo met at Forest Haven, the city's institution for the developmentally disabled, where both lived for years. They began dating when they were teen-agers. Before marrying they attended classes in family counseling and financial management.

Donna Thornton is training to become a houskeeper at D.C. General Hospital. Ricardo Thornton works at the Martin Luther King Library.

To prepare for the birth of their child, Donna Thornton is attending parenting classes at D.C. General Hospital and Ricardo Thornton will soon join her.

"When the baby's a little older, we'll tell him a little story about his parents," said Ricardo Thornton. "We'll even tell him being retarded doesn't mean that much.

"This is just another challenge," he said.

"A nice challenge," said his wife