PITTSBURGH — A man is not being allowed to see his three children because he had a sexual relationship with a 16-year-old — a relationship that he says was legal, but a county agency called sexual abuse, according to a lawsuit.

The American Civil Liberties Union of Pennsylvania, which represents the man, is trying to prevent Fayette County Children and Youth Services from placing his children in foster care.

The man is identified only as John Doe. His wife is in a state mental hospital. His children, ages 8, 6 and 5, are staying with his parents.

David L. Madison, the agency's administrator, would not comment.

According to the suit, which was filed in federal court:

The man briefly had a consensual relationship with a 16-year-old girl nearly two years ago when he was 27 and separated from his wife. He and the girl had deliberately waited until the girl turned 16 to start having sex, because they knew that was the age of consent in Pennsylvania.

Upon learning of the relationship, the teen's mother went to police, who told her nothing could be done because the girl was old enough to consent. The mother then told the children and youth agency that the man had abused her daughter.

In September 2006, the man sent his children to stay with his parents during the agency's investigation. He was permitted supervised visits until August, when a caseworker told him he'd been "indicated as a perpetrator of sexual abuse," and that supervised visitation was prohibited.

He was informed he could not have contact with his children unless he successfully completed a sexual offender treatment program.

He attended one class, but refused to admit — as required — to being a perpetrator of sexual abuse. He refused because he disputed the agency's claim that his relationship with the girl was sexual abuse.

The agency informed him that because he wasn't participating in a sex offender's program, "it is our policy that you can have no contact with any child."