You're confusing the Regnerus studies with other similar ones. Regnerus didn't try to establish how many of those kids were gay, only to document their behavioral problems.
But his study didn't even measure that accurately, because he was comparing children who had a mother and a father for 18 years with kids who were NOT raised for 18 years in homes with committed same-sex couples as parents. They were children who had experienced foster care, single or divorced parents and blended families. The most common age at which kids reported living with a mom and her partner was 14, so it’s unclear what kind of family structure these kids grew up in from birth to age 13.