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Tight pants and underpants:
The Gemara implies that it is forbidden for men to wear pants unless they are made like Batei Shukayim. [This however only refers to pants that are made with a special pocket that is tightly fitted for the size of the Eiver, and hence can cause one to be aroused. It also includes a prohibition against wearing pants that are tight around the Erva area due to the friction and arousal that they cause. However regular non-tight pants that we wear today are not included in this prohibition at all, and it is obvious that throughout all generations people did not walk without pants.]
It is forbidden to wear tight fitted clothing that cause friction and arousal to the Erva area.
May one wear underwear, boxers and the like?
It depends on the tightness of the fit. Tight fitted underwear is forbidden to be worn due to it causing arousal. The same would apply to tight fitted boxers. However baggy boxers and loose fitted underwear may be worn. It is customary amongst many Chassidic Jews to wear loose boxer style undergarments rather than underwear due to the above reason.
A Jew may not wear clothing of gentiles which give off a sense of arrogance and immorality. One is to wear clothing that express humility and modesty. Black clothing are proper to be worn as they express this matter. Red clothing are not to be worn.
May one wear jeans?
It is customary among many G-d fearing Jews to avoid wearing jeans. Tight fitting jeans, as well as any tight fitting clothing, are prohibited by some authorities due to them shaping the body and hence bordering “clothing of immorality/pritzus”. Those jeans that are tight fitted around the Erva area and cause friction and arousal to the Erva area are forbidden to be worn from the letter of the law, as explained above. Regarding jeans that are a loss fit, seemingly they are avoided from being worn due to them being a workers clothing and hence lack Torah elegance. Likewise even baggy jeans have come to signify a more modern life style in the eyes of the onlookers and hence are avoided by many G-d fearing Jews. Nevertheless there are G-d fearing communities in which certain shades of jeans are an accepted code of dress and hence no judgment can be passed.
 Rama E.H. 23/6
 Perek Kol Hayad
 Batei Shukayim are legging shoes that would cover the feet and reach up until the thighs. It would not cover the Erva area. These are similar to knee-highs. [Meir Oz 128/5-4]
 The reason: As this can lead to spilling of seed. Now, although one can explain the Gemara to only refer to their times when they ate Teruma, and there is thus worry that one’s body may become impure, nevertheless from the fact the Rosh records this ruling it implies that it is forbidden even today. [Rama ibid]
 Pischeiy Teshuvah 23/4 in name of Atzei Arazim; Aruch Hashulchan 23/7
 Ezer Mikodesh 23
 Pischeiy Teshuvah 23/4 in name of Atzei Arazim; Ezer Mekodesh 23; Aruch Hashulchan 23/7
 Many leading Rabbanim in Eretz Yisrael have publicly come out against wearing tight fitting pants due to this reason. Others however claim there is no source for prohibiting tight fitting clothing by men, although it may not be a proper clothing to wear.
 See Igros Kodesh 18 p. 380; Hisvadyos 1983 2 p. 759; Shulchan Menachem 1/5; 4/1.