The naturalistic fallacy — an appeal to "natural law" — was a common fallacy employed in David Berg's doctrines.
This fallacy falls under the more inclusive set of fallacies known as argumentum ad antiquitam, or, in English, an "appeal to tradition" and was commonly employed by Berg and The Family to characterize sexual impropriety as "natural" on the basis of average ages for puberal developments (i.e. that sexual development often commences prior to the age of consent).
The argument neglected to note the discrepancy between sexual maturation and psychological maturation, and was self-indulgent in naïvely believing that sating adults' urges to have sex with minors would not be harmful.
For more information on this logical fallacy see the link(s) below.