If you look at the exit polls on CNN, they classify who voted and how they voted.
They have lots of categories (age, race, church attendance, etc), but the most basic and simplest category is gender
Voter Gender Who they voted for
Clinton Obama
Male 41% 48% 52%
Female 59% 57% 43%
Based on the above, Clinton got 0.4148 + 0.5957% = 53.9%, and Obama got 0.4152% + 0.5943% = 46.7%, so these exit poll numbers are fairly close to the final result.
The numbers clearly show that women were much more gender-biased than men.
Men only slightly favored the male candidate (52% to 48%), while women strongly favored the female candidate (57% to 43%).
That alone would have put Clinton over the top, even if equal numbers of men and women voted. (it would have resulted in 52.5% for Clinton and 47.5% for Obama)
But, in addition to that, more women came out and voted (by a large margin: 59% to 41%), thus ensuring the large gap we saw in PA.
By the way, given the strong gender-bias of women voters in PA, men would have had to come out in record numbers to vote (77.8% men vs 22.2% women) in order for Obama to come out ahead.
So, while all the pundits and people here on the SDMB are trying to explain “why Obama couldn’t close the gap in PA” using complex analysis, it seems to simply be the strong gender bias and strong turnout of female voters (at least in PA)