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It probably has to do with outgoing links as well. Your examples have hundreds or thousands of outgoing which could have an effect despite thousands and thousands of incoming.
Those two sites are also trusted by google and probably get extra favor for it, at least from some Matt Cutts quote a while back.
An equally trusted adult site, which was not a link site, might have a higher pr. Then again, i doubt they would rank higher for se results than gg and others.
fwiw i have seen adult sites that had a pr 7 or so. Clickhereforxxxpix had a 7 i believe.
Brad
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