You should read the study if you want to know why it was done the way it was done. Scientists are very good at explaining why the criteria they selected are the criteria they selected.

From the summary report

This report is part of an international survey that included men in Australia, the US and the UK, where the age of consent varies from 16 – 18 depending on the region and the offense. Accordingly, we used 18 as the age of consent for all sexual offences, although the age of consent for sexual activity is 16 in Australia. Therefore, some of what is noted as sexual contact with a child in our findings may be consensual activity (for instance, a 19 year old having sex with a 17 year old), which is a limitation of the survey. However, it is unlikely that our contact offense findings were significantly impacted by this limitation due to the following factors

Our survey included a question on whether participants had intentionally viewed “pornographic material” containing people under the age of 18 (that is, child sexual abuse material). Participants could answer whether they had never done so, had done so when they were under 18, or had done so when they were over 18. Since this was a forced option, those who accessed child sexual abuse material as a child, but continued offending as adults, may not have been included in the adult offending analysis. As a result, our data on child sexual abuse material offending may be an under-estimation.