I don't know if I actually believe this, but I've heard reports that cause me to assign a non-neglible probability on the chance that sexual relations with between children and adults sometimes don't cause harm. For instance, see the Rind et al. report:

"Child Sexual Abuse does not cause intense harm on a pervasive basis regardless of gender." Simplified, Rind et al. (1998) found that 3 out of every 100 individuals in a CSA population had clinically significant problems (compared to 2 out of every 100 in a general population).

Rind et al. contended that the degree of psychological damage was based on whether the child describes the encounter as consensual or not.

It's hopefully obvious that this is not a justification for having sex with children, even if there is one controversial study suggesting it might sometimes be okay.