Your question presupposes a positive answer -- you should be asking, "Are American morals and values in a state of decay? Why or why not?"
The real issue with this question is that morals and values are subjective. Obviously, I'm sure we all agree that violence, political corruption, dishonesty, and cruelty are wrong. Things like rape, murder, and torture can never be excused -- unless, of course, it is the enemy. War and suchlike. (disclaimer: I do not agree with the previous statement; it is an example of how values are subjective.)
Which perfectly illustrates that all morals and values are subjective -- to societal mores, upbringing, etc. Person A may find the idea of sleeping with anyone other than their spouse repugnant, but divorce a reasonable solution to an unhappy marriage. Person B, on the other hand, may find the idea of sleeping with other people (with permission) a viable means of adding spice to a relationship, but the idea of divorce is appalling and horrific.
I myself do feel that modern society is having a decline in our values -- mainly, in our educational values, dedication to open-mindedness, and willingness to engage in honest, self-questioning discourse. It seems to be a trend right now to always insist on absolutes, without entertaining the possibility of personal or cultural mistakes.