If you look at Sota 7-8, and Sanhedrin 45, you see that the Cohein ripped the top of her dress to expose her heart. (The Torah tells you to do this, among other things) And then he ties a string (reed) around the top of her breasts so the dress shouldn't fall off. This whole thing is done to embarrass her, and the ladies must see this and learn a lesson. And the men may also see this. Rabbe Yehuda said "if her heart (Upper section) is pretty, you don't rip her dress" because she may go free, and the teenagers who saw her undressed may run after her (For sex). Rabbe Yehuda can do this through 'shev veall taaseh'.
The Talmud then goes into a whole thing about why Rabbe Yehuda lets a lady get stoned to death, and she only wears a minimal front and back cover like a 'g- string', and he's not worried about the teenagers getting all horny, and running after other women. "The Yeatzer Horah's power is only on something that you saw with your eyes", and since the lady that they saw naked with their 'eyes' is now dead, they will not run after other women that they did not see naked.
When it comes to porn it gets rather complicated. You're not gonna say that if I watch a porn movie and then I destroy it, I will never think about it again or will you? Tosfos talks about Rochov the Zonah who was one of the most beautiful women in the world (Who ended up marrying Yehoshua) who was so sexy that if you knew her, all you had to do was say "Rochov, Rochov" and you ejaculated immediately. So here you see long distance lust even though she was not around. However, we are also talking about someone who had sex with her, while the teenagers had nothing to do with the dead lady.
The Maharam says that of course Rochov was very pretty, and it basically boils down to how strong is the Yeatzer Horah? You don't have to be a genius to figure out that today's society is flooded in sex, and some people can even ejaculate just by looking at a tree. So the bottom line is that it's an Essur Deoraysa to have thoughts by day, which bring emissions by night (Avoda Zara 20b), and you gotta use your head whatever you do.
Oh! I forgot the main point. Jewish women get very embarrassed if you uncover the whole neck line, from the neck down to the breasts. So the next time you go to a modern wedding and you see a bunch of Sotas walking around............