That number is based upon the estimate that 40% of the current crop of priests are homosexual. There is no factual evidence for this, just 'gut feelings'.
It's based on several different questionares sent to Priests. If gay priests were represented in the church in the same percentages as in society, it wouldn't be an issue, would it?
My gut feeling is that the 25%-50% figure does not come from reliable studies, and should not be cited. But that's just a feeling. I don't pretend that it is a fact.