SAN FRANCISCO - A California gay couple sued the Internet site Adoption.com on Tuesday, alleging the site won't help them adopt a child because they are homosexuals.

Michael and Richard Butler of San Jose, who legally changed their last names so they would be the same, have been domestic partners for about eight years and share a house, according to the suit.

After deciding to adopt and being certified by California to do so, they sought the services of Adoption.com, which lists the profiles of hundreds of birth mothers as well as qualified persons wanting to adopt.

The suit, filed in San Francisco County Superior Court, alleges the site has unlawfully rejected the Butlers' application to join the site.

"This is a clear-cut case of discrimination," said one of the couple's attorneys, Neel Chatterjee.

A spokeswoman for Tempe, Ariz.-based Adoption.com declined to comment.

The site's legal director wrote in a letter to the National Center for Lesbian Rights, which is suing on behalf of the Butlers, that heterosexual couples are better suited to adopt than homosexuals.

"All of the scientific research that we have seen overwhelmingly supports the premise that children have a much greater chance of thriving in their lives ... if they are raised in a stable and traditional two-parent family," said the letter, which was included in the suit.