But this isn't double blind.

You mean there is no control group. Since this is a report from the field, not the laboratory, "blind" or "double blind" does not apply. It would be difficult in any case, as in a blind study the participants do not know which condition they are in. It is hard to imagine a study of sex as we normally think of it where the participants don't know whether they had sex or not.

How many retroactively regret not to have had sex earlier?

I don't know. I've not heard any anecdotal reports along the lines of, "If only I'd agreed to have sex with Mr. Jones when I was 10, I would have been sexually fulfilled and life would have been so much better!" It certainly could be tested with questionnaires.

Isn't it so convenient to demand telepathy knowing full well it's an impossible standard to meet?

It certainly can be studied by interviewing men and girls independently about the same experiences. I'm not aware of published research on the subject, but certainly have heard many anecdotal reports, including from Tom O'Carroll. Most of Clancy's subjects lacked any sort of positive consent. I imagine many of the men thought they had consent, though I don't know for sure.

A key component of sex offender therapy is "empathy training". That may be concerned with getting men to care whether the girls are going to be harmed or not, but also would include understanding that a failure to object or struggle is not enough to mean consent.