"When B'nai Yisrael went down to Egypt, they behaved with modesty - each person living in his own tent, as the verse says (Shemot 1:1): 'Each man and his household came.' Reuven did not look at Shimon's wife, nor did Shimon look at Reuven's wife. Rather, each man lived modestly within his own tent. Even when the population of men numbered 600,000 in the desert, not one man placed the opening of his tent opposite the opening of his friend's tent." (Yalkut Shimoni, Balak)
In ancient days, life was different and people were different too. In the modern world, we are so used to seeing women in public that we have grown somewhat "immune" to them. So shouldn't these laws and teachings be modified?
Yes, times have changed, and there are some leniencies in the fine details of some of these laws. Please consult your rabbi for specific questions and rulings. The four laws that we learned in Day 7, however, are d'Oraitah, direct Torah prohibitions, and immutable.
A standard rationale offered by those who are lax in Shmirat Einayim is the idea that we've grown so used to seeing women immodestly dressed in public that it barely affects us anymore.
One of the greatest Ba'alei Mussar was Rabbi Eliyahu Lopian, who taught Torah for over seventy years in Europe and Eretz Yisrael. One of his students was invited to a wedding where the laws of modesty would not be observed. He asked Rabbi Lopian if he could go to the wedding. When asked what he would do about the lack of tzniut, the student told him that the sight of immodest women does not affect him. Without another word, Rabbi Lopian reached for a book of Tehillim and started praying. "Rabbi, what are you doing?" asked the student.
His teacher responded, "I am 86 years old and blind in one eye, and I am still affected by human nature. You are young and in the prime of life. If you are not affected by immodest sights, then perhaps you're sick. I am saying Tehillim for your recovery!"