I have a question from another post here.
If pants are considered to be immodest garments in general, aside from the issue of them being a male garment, why are they permissible for men to wear? Are they considered immodest just for women? Why? "Pissuk Raglayim" (spreading the legs) has been mentioned as a reason, but why is it less modest for a woman to spread her legs than for a man? After all, men have things that women don't and vice versa.
Although the real answer to your question would be found in the Double AA's answer, it's worth noting that there was at least one authority who did believe that pisuk raglayim was an issue for men as well, albeit not as big an issue as for women. The Chazon Ish felt that wearing a long jacket was proper for exactly this issue. In fact he would wear an extra large tallis kattan so that when he went on his health walks in the heat, he would be able to drape his jacket over his shoulders and still have the tallis kattan cover enough of his legs. This is reported by Rabbi Aharon Roter author of Shaarei Aharon who was the caretaker of the Chazon Ish towards the end of his life.
It seems to me that the answer to your question is that a man's spreading his legs is not as suggestive a position as the posture isn't reminiscent of intercourse (where a woman's spreading her legs is more...ergonomic).
Indeed the phrase פישוק רגליים "spreading the legs" comes from Ezekiel 16:25 where the context and commentaries are clear that what is erotic is her assuming a posture she would take during intercourse.
There are different types of trousers manufactured today.
Some are designed to be loose fitting, and modestly cover the crotch area, even if the man is sitting. However there are some types of trousers that are especially cut, in a manner which accentuates the crotch and the male organ. These latter styles are considered vulgar by thoughtful people who are sensitive to modesty.
In the days of old, even non-Jews were sensitive to this refinement, and therefore wore tunics or robes that covered the crotch area. However in today’s times, decadence has prevailed, and clothing designers design trousers in a manner that is designed to call attention to that which was once considered to be modest and private.
While the man’s organ is technically covered, and halachically, berachos and prayers may be recited in his presence, nevertheless such types of trousers are vulgar, and are not befitting to be worn by those who seek to dress in a dignified manner. While this concept is obvious to sensitive people, those not attuned to it will view it as extreme.
We find that in the holy temple, the altar had a ramp on it, via which the priest would ascend to offer the sacrifices, not steps. The verse (Shemos 20:23) notes that the use of steps would be tantamount to revealing ones nakedness to the holy altar. Rashi (ad loc.) asks, being that the kohanim wore trousers, how would their nakedness be revealed when using steps? Rashi answers, that the mere stretching of the legs which is required when using steps, is considered to be close to revealing one’s nakedness, even when fully clothed. Such was the refinement which was called for in the setting of the holy temple!
The lesson for us in this is to understand, that the revelation of the crotch area, even if covered, is a step down in modesty.
