UJM – i disagree; while of course you’re right that more ervah is shown by short skirts, and that will he a bigger michshol for men, i think the harm that wearing pants does to a woman’s perception of yiddishkeit and the tzurah of a bas yisroel might outweigh that factor – i think it would take a gadol b’yisroel to decide such a thing. Pants are a pirtzah in that they are a different category of attire altogether foreign to the Torah’s paradigm of jewish femininity; with a shorter skirt you can always change to a longer one. From the perspective of a man trying to avoid shmiras aynayim pitfalls, this is irrelevant, but let us not forget that tznius impacts women as well, being their defining characteristic, as the gra explains in his famous letter.
If a rov would advise women who were unwilling to lengthen their skirts that they’re better off wearing pants, the results wouldn’t be longer skirts, it would be the birth of a completely new pirtzah , along with all of its roots in feminism and its own unique tznius issues (i.e. form fitting pants) that heretofore were not an issue. Do we really want to internalize a new nisayon for our already battle weary bnos yisroel? “Miri wears pants…her rabbi said she’s better off doing that than wearing short skirts…must be pants aren’t so bad” will be the sentiment of many, many young women.
It can also be argued that the actual gain that men would have from this proposition wouldn’t be that much, since they would still encounter short skirts, as thus proposition will be no less enforceable than our current guidelines…
So a shailah posed to a gadol would be, which is more important? A possibly significant – although hard to predict – mitigation of erva for the benefit mostly of men, or the core change and corruption of the values of jewish dress, kedushah and identity on the part of the women? As my tone implies, I believe the latter.