Parshas Metzora deals with various types of tumah. The Kuzari (2:60) explains that all forms of tumah are in reality offshoots of the primary tumah - the tumah of a corpse (See also Ramban in this week's parsha 15:11 who mentions this idea). Chazal tell us that the metzora is likened to a dead person. The tumos of niddah, zivah and shichvas zera are all created by bodily discharges which could have contributed to the creation of a living being. A lack of creating life, in effect, constitutes a form of death.
Rav Wolbe explains (Daas Shlomo) that death is a phenomenon that was not built into the fabric of creation. Only after the advent of cheit in general, and the sin of eating from the eitz hadaas in particular, did death become the way of the world. Thus, cheit and tumah are closely related since cheit caused death and death is the root of all tumah. For this reason, the Torah imposed laws that necessitate various levels of distance that must be maintained from those who are tamei, since the Torah wishes that we distance ourselves from sin and all its consequences.
Chazal tell us, "Sin covers over the heart of a person" (Yoma 39a). Every sin causes the heart to be covered with a thin film of impurity that dulls its innate sensitivities. This dulling of the senses is similar to death since, to a certain degree, the clarity of the heart's perception ceases to exist.
Kedusha stands diametrically opposite tumah: it signifies life and it allows one to experience life in its truest form. Moreover, it instills one's heart with sensitivities that are unknown to those who lack his level of kedusha. Such a person can sense a cheit that might have gone unnoticed by someone who does not possess such a high level of kedusha.
Every sin obstructs the spiritual arteries of the heart thereby dulling its ability to discern right from wrong. Not being offended by seeing an indecent sight is not something to be proud of. One of our great leaders compared it to a peasant who walks barefoot on pebbles without it affecting him: Both have simply become desensitized to the point where things that should set off bells simply go unnoticed.
Indeed, there are many things that while they do not affect gentiles, they definitely have a negative effect on Jews. We are aristocrats and we can sense even the smallest deviation from kedusha. Appreciate your innate greatness and guard this virtue as you would the apple of your eye!