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.