It's based on multiple studies compiled over the years. You can view them all here on the summary page: http://www.familystructurestudies.com/summary

EDIT: Hmm, looks like a lot of the articles attached to those studies were conveniently removed, but you can still access the PDF about the lesbian/gay child abuse statistics here : https://www.acf.hhs.gov/sites/default/files/documents/opre/nis4_report_exec_summ_pdf_jan2010.pdf