Germany's highest court has upheld a law that makes incest a criminal offense, rejecting an appeal by a man who was sentenced to prison after fathering four children with his sister.

The Federal Constitutional Court said Thursday it has ruled that the state is within its rights to cite "the protection of family order against the damaging effects of incest ... and the avoidance of serious genetic illnesses" in outlawing incest.

The ruling followed a complaint by a 31-year-old man from eastern Germany, who has been identified only as Patrick S. He received a 2 1/2 year prison sentence for incest in 2005 but has been free pending the supreme court ruling.

His sister, Susan K., who is now in her early 20s, was placed under the supervision of social services.

The man had been given up for adoption at age 4 and met his birth-mother for the first time in 2000, at which point he also first met his sister. He and his sister then had four children.

The man's lawyer has argued that there is no reason why two people who love each other should not be allowed to live together — purely because they are siblings.

However, the constitutional court ruled that sexual relations between siblings "do not affect them exclusively, but also can have an effect on family and society, and have consequences for children who arise from the relationship."

The court said one of the judges who considered the case dissented in the Feb. 26 ruling, arguing that the ban was disproportionate.