'Now why should trousers offend Him, but short hair not?'
A word about hair...
'Doth not even nature itself teach you, that a man indeed, if he nourish his hair, it is a shame unto him? But if a woman nourish her hair, it is a glory to her; for her hair is given to her for a covering.'
--1 Corinth. XI:14-15
'Even if that were true, it doesn't make them immodest.'
Regardless of whether or not one personally considers women wearing pants immodest, it is still wrong.
'Tell anyone to stop doing something which isn't wrong, and see the reaction!'
But, it is wrong. That's the point.
'I have seen very feminine outfits which include slacks and cannot see the problem with them.'
This doesnt surprise me.
'I do not see that there is an inherent problem with women's wearing slacks.'
I dont have the desire to go back through 22 pages of banter, so I'll just assume that Giuseppi Cardinal Siri's letter has probably been brought up. However, I will post the following nonetheless:
'However, it is a different aspect of women's wearing of men's trousers which seems to us the gravest. 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.'
'We do not deny that modern life sets problems and makes requirements unknown to our grandparents. But we state that there are values more needing to be protected than fleeting experiences, and that for anybody of intelligence there are always good sense and good taste enough to find acceptable and dignified solutions to problems as they come up. Out of charity we are fighting against the flattening out of mankind, against the attack upon those differences on which rests the complementarity of man and woman.'
'When we see a woman in trousers, we should think not so much of her as of all mankind, of what it will be when women will have masculinized themselves for good. Nobody stands to gain by helping to bring about a future age of vagueness, ambiguity, imperfection and, in a word, monstrosities.'
I dont know if that counts as infallible: probably not. But, very authoritative and clear coming from a Catholic cleric pretty high up on the food chain.
So, again, you can try to justify to yourself whether or not women wearing pants is immodest. The point is that it is wrong for women to do so, generally speaking. Therefore, the modesty question is irrelevant.