The Gemara [Shabbos 64a] relates that the soldiers who came back from the revenge battle against the Midianites offered the gold and jewelry they captured in battle as a penitential tribute, because, even though Moshe initially suspected them of illicit relations [precisely because they offered the booty under their own volition], they said “’We did not do a sinful action, but perhaps we had sinful thoughts’…they taught in the House of R’ Yishmael: the soldiers needed [to bring the Kelim for] an atonement for getting pleasure from seeing forbidden women.”
This is one of the myriad Talmudic sources detailing restrictions regarding ogling—or, more likely, gazing that would fall far short of ogling.
Sefarim do exist that actually explicate “hilchos re’iyah” in great detail. Suffice it to say it’s not one of my strong points. It reminds me of the story a former shana bet colleague told me about the time he and a chavrusa, having been able to make the commitment to be “shomer negiah” and stick to it [more or less, at least for a measureable period of time], wanted to go the next step and try to be “shomer re’iyah”. According to him, that might have lasted a few minutes.
[There is also guardyoureyes.com, which involves a “battle” that didn’t really exist yet in my Yeshiva days. [But I date myself.] Should I mention the irony of a website that helps one combat the vagaries of the Web? I just did?]
This vignette is also usually a jumping-off point to most summer-period mussar shmuessen about watching what you—well, watch. But it might not present as the paradigm of self-control it’s held up to be when you take a closer look.
This battle was the quid pro quo for Avel Shitim, the joint Moabite-Midianite turning-out of almost their entire flower of young womanhood to seduce Bnei Yisrael into Pe’or-worship and concomitant oblivion [cf. Sanhedrin 106a]. [Not-so-side note: the mass executions taking place during the incident were for the Pe’or worship, not the illicit liaisons; the only exception was Zimri, and there were other reasons for that.]
There are midrashic indicators that soldiers weren’t too far removed by degree from the precipitating factors. Rashi on 31:16 via Sifri relates that the soldiers actually recognized which particular woman had snared which particular Israelite offender: "this is the one through which so-and-so had his downfall." [Since, as the aforementioned Sanhedrin 106a relates how Bilaam planned out the gradual seduction technique through business relationships, personal knowledge of at least who some of the participants were may not have been all that farfetched.]
Tosfos Shabbat 64a [s.v. “midei hirhur mi yatzanu”] actually indicates one more interesting tidbit: the halacha of the “yefat to’ar”/captive woman was already known to the soldiers, but a reminder was needed that it applied to “the one you liked [“ve’chashakta”], but not her friend”. [I’m unaware of anything indicating whether anyone actually made use of the loophole, but this Tosfos indicates that at least there was some consideration on the part of the soldier which would have been halachically legitimate, insofar as the boundary as to where it would not have been was delineated.]
So, not to necessarily downplay “lo sasuru”, but there were plenty of cues to hirhur here which made the situation other than a case of “re’iyah be’alma”. Hardly paradigmatic, then.
Beyond that, however, is the extension that this might take to another realm that is the complete inversion of this case. I can’t locate where I saw this, but a [well-meaning?] advocate of more sex-segregation in religious activity justified his position because, as he put it, he might be having a “spiritual moment”, which is then ruined by a female either singing or giving a dvar Torah.
To said complainant: if your “spiritual moments” are a] all about your having them and b] they are so fragile that they dissipate when a woman might simultaneously engage in something [at least] equally legitimately spiritual in the time and place where you are—then you probably have to reassess either what you call “spiritual”, your actual “spirituality”, or both.
This notion has actually been addressed in the contemporary literature. In his “Understanding Tzniut”, Rav Yehuda Herz Henkin takes issue with a lot of what’s written in Rav Pesach Eliyahu Falk’s “Oz Vehadar Levusha” [I’ve heard that at least one BT women’s seminar tells their students to particularly NOT use that sefer as their tznius arbiter]. Or, as Rav Henkin puts it, it’s a sefer “as much about ideology and outlook as [it] is about halacha.”
Most germane to this issue—and the aforementioned complainant—is where R’ Henkin addresses R’ Falk’s approvingly relating the “rebbetzin who never displayed her vast knowledge…she would listen quietly and closely as if the words were new. She never hinted that she was fully acquainted with what was being quoted.” R’ Henkin: “An alternative—that she should share her knowledge with others and deliver a d’var Torah—is not considered. Apparently, that would be displaying special skills and a lack of tzniut.”
And, the coup-de-grace: “There is a danger here of losing sight of the real basics of modesty—not to mention being so concerned about not thinking about women that one can think of nothing else.”
Res ipsa loquitor.