lilmod ulelamaid – For a serious answer (and not Joe, which avoids the question), you first need to understand “Tznius”. I’ll quote myself:
As promised:
There are THREE (really four) different aspects of what is known as “Tznius” (which is like calling lighting a fire on shabbos “Mukzta”). They are:
1: Ervah
2: Das Moshe/Yehudis
3: Tznius
4: Histaklus
1: Ervah is a din in Kriyas Shema (which is why the halchos are in Keriyas Shema). Many Rishonim hold it applies to other brachos as well. The idea is that a Davar Sheb’kedusha may not be done/said before any Ervah. That includes for a man (as per the Gemorah Brachos) a woman’s hair, shok, etc. This has NOTHING to do with the general idea of a man looking at a lady. See Shulchan Aruch, Orach Chaim 75 for more details.
2: Das Moshe/Yehudis is what the Torah (add: or Chazal) perscribes that a female should not have uncovered when she goes out in public. This includes hair of anyone who is not a Besulah (as per Even HaEzer 21:2). The Mishna in Kesubos (72B) differentiates between the two. Ayin Shom. (Rav Moshe IIRC also has some teshuvos on the matter).
3: Tznius: There is a general concept of Hatznya Leches. This applies to both males & females, both in manner of dress and action, to be “low-key”. (There are also halachos of Tnzius in OC 240, which I will ignore for this discussion). An example is a woman covering her hair in her own yard (a private domain), where there is no Chiyuv (seemingly even if others will see her), but there is a concept of “Tznius”. (Offen a Bais Shemuel in Even HaEzer). In general, Chassidim are more Machmir in this.
4: Histaklus: A man may not stare at a woman. Period. If he does, he is Over “Lo Tasuru”. It certainly is a good thing for women to make sure that men have no reason to stare at them, but unless it is “Trai Ivri Nahara”, then it is not “Lifnei Iver”.
Depends on the scenario. Let’s say a woman is sunbathing in her own backyard, where there is no Chiyuv to be covered, but men can see (for whatever reason). The men have a chiyuv not to look (Etzba Ketana and Histaklus), but the woman has no chiyuv to cover up (as she is in her own chatzer).
Does that answer the question?