"Presumably they are having sex," said Tess Beck of New Haven, Conn., whose 31-year-old son has Down syndrome, who previewed the film.
Although their love is genuine, the consideration of children seems "irresponsible," she said.
"There is no reason to stop them from loving one another and marrying, as long as the fertility thing is figured out," she said.
Beck's son Christopher lives in a group home in Vermont and has a girlfriend, but she worries that marriage might be too much for the couple to handle. "It would have to be carefully thought out."
As for the film's couple, "It's lovely that they could get married and be happy together," said Beck. "But there is nothing normal about their life. Their parents are giving them all the good parts and none of the responsibility."
Few statistics exist on adults with Down syndrome -- even on important questions like their sexuality, according to Emily Perl Kingsley, a veteran writer for Sesame Street and an activist for those with disabilities.
Biologically, women can ovulate, though only about 50 percent are fertile. Information on men is less clear, mostly based on studies of boys in institutions in the 1930s that concluded they were sterile.
According to NDSS, there have been two documented cases where the paternity of a man with Down syndrome was confirmed.
Kingley's son Jason, now 36, exceeded all expectations, appearing at the age of 3 on the pioneering children's show. But when he was born in 1974, Kingsley's obstetrician urged her to institutionalize the baby.
"He told me that my child was going to be profoundly retarded and never walk, never talk and would never read or write and or be able to distinguish us from any other adult," she said. "He would never have any meaningful thought process...He told me to go home and tell my friends he died in childbirth."
Many children of that era lingered in institutions, dying of respiratory illnesses before they were 5, because they were never given antibiotics.
Others died of heart conditions before modern medical advances. An estimated 30 to 50 percent of children born with Down syndrome are born with heart anomalies, according to Kingsley.
"They were not considered worth treating," she said.
But a social worker suggested a new concept, "early intervention," and Kingsley decided to keep Jason at home.
There, he thrived, learning to read and appreciate classical music, as well as getting a high school diploma. With a co-author, he wrote "Count Us In: Growing Up With Down Syndrome."
Today, Jason is in good health and lives in a structured group home with two roommates who also have Down syndrome. Though he is "damn bright," according to Kingsley, he has "significant gaps" in knowing how to "manage in the world."
While his dream to direct animated films is probably ambitious, Jason's work in a mail room is "well below his capabilities," she said.
"There's so much stereotyping," said Kingsley. "They are wiping down tables at McDonald's or bagging groceries at Shop Rite. Not too much else is offered."
After a sexually "complicated" relationship with a girlfriend, Jason had a vasectomy.
"I wish it for him to fall in love," said his mother. "I think we tend to infantilize [adults with Down syndrome] and think that they are not capable of real emotion. But that's not true."