Before we approach the issue of "darcha acharita", avoiding temptations, let's examine the Halakhic parameters of two prohibitions.
1) Hirhurim … sexual thoughts. Many think that it is prohibited for a man to experience any such thoughts whatsoever about any woman (other than his wife, obviously) ever. Well, I don't know what is and what isn't prohibited. I only know what others have said. Both the Me'iri and the Ezer Mekudash (quoted in Otzar HaPoskim 23:3:8:2, for your convenience), both frum yidden, maintain that it is prohibited to engage in purposeful and intentional sexual fantasy. Occasional thoughts are not prohibited, as long as not purposeful and intentional. So guess what, gents. Chill. You may, on occasion, experience some level of inappropriate thoughts. It may be the way men were built, an issue which would require another post by a social biologist. In the words of the Ezer Mekudash, "the Torah was not given to angels." You are not angels, gentlemen. Get over it. You are not obligated to decimate such thoughts, even when they occur, on occasion, inappropriately. A simple victory by "fall" (to use a wrestling term) is all that is required. Do not act on these desires, including … no purposeful intentional sexual fantasy. Beyond that, there is no Halakhic obligation to be overly focused on this challenge. Don't obsess. R. Yisrael Salanter (אגרות ומכתבים מאת ר' ישראל סלנטר, מהד'שרגא הכהן וילמן, סי' כה ) wrote that obsessing over these thoughts can act as a boomerang and make them even stronger.
2) Seeing/staring at/ogling a woman. Halakha does not prohibit seeing any part of a woman other than the genital area (see Shu"t Naharei Afarsimon, EH 19). The poskim distinguish between seeing and staring. This distinction is found all over the place in the Halakhic literature, but if you can't find it, then hit the books, get back to the beit midrash, and in the meantime, take a look at Yad Malachi (179) since that's the source that comes to me off the top of my head. Staring at part of a woman defined as "erva" is prohibited. Staring at parts of a woman not defined as erva is not prohibited, as long as there is no intent to derive pleasure. Ogling, that is, intentionally deriving pleasure from staring at a woman, is prohibited even if ogling the woman's small finger. Stop worrying about what you might see in the bus, at work, or elsewhere. I have NEVER seen a woman in public whose genital area is exposed. Don’t stare at a woman who is not dressed in accordance with normal social standards (see below). And never ogle. Get a grip. The distinction between staring, with no intent of deriving pleasure, versus ogling is to be found in the Rambam, Bi'ah 21:1 versus 21:21.
It is important to note that societal norms determine what parts of a woman's body are defined as erva. If you live in a society where social norms sanction mini-skirts, belly shirts and halter tops, then areas of the body exposed by such clothing are not defined as erva. Sound too modern? Take a look at Dirvei Hamudot (Brackhot 3:116 ) who writes that the upper arms of women are considered erva only in a society where women normally cover them. Well, maybe that only applies to a lady's upper arms? Guess again. Yaskil Avdi (vol. 4; EH:9 ) writes that in a society where social norms sanction exposed breasts they are not considered erva (click here and see the last paragraph and continue to the next page).
The degree to which a man is vulnerable to "immodest" dress is directly proportional to the degree of modesty to which he is accustomed. A man who lives in a haredi neighborhood and never leaves it is most vulnerable to dress codes less modest than those of his neighborhood. A man living in a haredi neighborhood who works in N. Tel Aviv (there must be at least one such person in the country) is less vulnerable. A man living in a mixed religious / secular neighborhood is yet less vulnerable. This should be obvious, but for those who want a Rabbinic source, see Shu"t Siach Nachum 92. So … a dati-leumi man living in a city (as opposed to an all religious settlement) is considerably less vulnerable to temptations than his haredi or settler colleagues.
Most important, this discussion should be quite unnecessary. Common sense is a really good rule of thumb here. Sound not so frum? Take a look at the Ritv"a at the end of Kiddushin and see that he says exactly that. True, he does say it is improper (though not prohibited) for anyone other than a saintly gent to take this approach, but see Shu"t Bnei Banim, vol 4, article 2 here where R. Henkin broadens this approach to include the average person as long as within the standards of modesty to which said person is accustomed.
Click here to see more of R. Yehuda Henkin's writings regarding how societal norms play a factor in the halakhot of modesty.
All this being said, why shouldn't we just adopt the most stringent positions in issues of modesty.
The answer is twofold:
1) the boomerang effect mentioned above in the name of R. Yisrael Salanter
2) the cost to society … stay tuned
I hope to write on the following issues in the near future:
1) darcha acharita – avoiding temptation
2) the societal price paid in mehadrin approaches
3) unity or uniformity … do those singing the song of achdut have any clue what they are saying?