No harm done there.
His mom thought the relationship was so harmless, she sued the city for a million dollars for failing to protect her son.
http://seattlepi.nwsource.com/local/17184_vili04.shtml
Besides the harm to the boy, an unemployed high-school dropout father of two children who aren't much younger than he was when he was seduced by Letourneau, we have 4 children without their mother, and two more children with a wacky grandma who did a piss-poor job of raising her own kids, lied on the witness stand during the criminal trial OR else lies to get $1 million from the city, an unemployed high-school dropout 21 year old daddy who can't get his sh%t together to act like a man and a responsible father to provide for them, and a convicted sex offender mother, who has a mental illness, and who still thinks getting it on with a little boy whose messed up home life rendered him starved for affection and ripe for the picking, as "beautiful".
Roses, it's just roses all the way around.
From a strictly scientific viewpoint this doesn't prove anything. We don't know if we would have experienced this or other problems if the relationship had never occured.