Notification Concerning Men's Dress Worn by Women, Cardinal Siri (1960):

"The first signs of our late arriving spring indicate this year a certain increase in the use of men's dress by girls and women, even mothers of families. Up until 1959, in Genoa, such dress usually meant the person was a tourist, but now there seems to be a significant number of girls and women from Genoa itself who are choosing, at least on pleasure trips, to wear men's dress [men's trousers - that is, slacks/pants]. The spreading of this behavior obliges us to give serious consideration to the subject, and we ask those to whom this Notification is addressed to kindly give this problem all the attention it deserves, as befits those aware of being answerable to God... The wearing of men's dress by women affects firstly the woman herself, by changing the feminine psychology proper to women; secondly, it affects the woman as wife of her husband, by tending to vitiate relationships between the sexes; thirdly, it affects the woman as mother of her children by harming her dignity in her children's eyes... In truth, the motive impelling women to wear men's dress is always that of imitating, nay, of competing with the man who is considered stronger, less tied down, more independent. This motivation shows clearly that male dress is the visible aid to bringing about a mental attitude of being 'like a man'. Secondly, ever since men have been men, the clothing a person wears conditions, determines and modifies that person's gestures, attitudes and behavior, such that from merely being worn outside, clothing comes to impose a particular frame of mind inside. Then let us add that a woman wearing men's dress always more or less indicates her reacting to her femininity as though it were inferior [to masculinity] when in fact it is only diverse. The perversion of her psychology is clearly evident. These reasons, summing up many more, are enough to warn us how wrongly women are made to think by the wearing of men's dress... Experience teaches us that when woman is de-feminized, defenses are undermined and weakness increases... The changing of feminine psychology does fundamental and - in the long run - irreparable damage to the family, to conjugal fidelity, to human affections and to human society. True, the effects of wearing unsuitable dress are not all to be seen within a short time. But one must think of what is being slowly and insidiously worn down, torn apart, perverted. Is any satisfying reciprocity between husband and wife imaginable, if feminine psychology be changed? Or is any true education of children imaginable, which is so delicate in its procedure, so woven of imponderable factors in which the mother's intuition and instinct play the decisive part in those tender years? What will these women be able to give their children when they will so long have worn trousers that their self-esteem is determined more by their competing with the men than by their functioning as women? Why, we ask, ever since men have been men - or rather since they became civilized - why have men in all times and places been irresistibly borne to differentiate and divide the functions of the two sexes? Do we not have here strict testimony to the recognition by all mankind of a truth and a law above man? To sum up, wherever women wear men's dress, it is be considered a factor, over the long term, in disintegrating human order."
 

"A woman shall not wear an article proper to a man, nor shall a man put on a woman's dress; for anyone who does such things is an abomination to the LORD, your God." (Deut. 22:5)

Cardinal Siri also offered the New Mass.  Not the best example of a man dedicated to Tradition. If he could be wrong on such a grave matter as the Holy Sacrifice of the Mass, why not on something as minor as pants?  Also, this is just one Cardinals opinion and is not an "official" condemnation of pants.

OK, then could you please point out the part in the quote from Cardinal Siri in which he erred, and show us how that error represents a departure from Catholic norms or femininity and modesty?

I don't think you (or anyone else) can.

PS: Please also be sure to point out where God erred in Deuteronomy.

If one does not naturally see the unsuitability of men's clothing on a woman as to their lack of feminine character and in many cases modesty, then you will be hard pressed to convince them.

Any man or woman who reads the following, and persists in promoting women wearing manly attire, does not have the Faith:


http://www.mycatholicsource.com/mcs/pc/catholic_life/modesty_dress.htm

Should have said "Pius X" not Luis "

That is utter nonsense.  A woman acts like a lady while wearing pants is still treated like a lady. Catholicism demands it and shame on those who do otherwise.  

Like I said, a woman who wares modest woman's slacks is not the same as a man in a bra and panties. Lets stay on track. Key words here are MODEST and WOMAN'S slacks.