This change can be attributed to a wide variety of factors, chief among them economic considerations. You also have abstinence education being taught in more schools. These kinds of societal influences will change the way people behave but will not change their nature, and if the culture reverts back to the way it was five years ago, so will the populace. As I stated earlier, culture and social norms highly influence the way people act, and what you have just provided is an example of that.
Doesn't matter WHY they changed. Only matters THAT they changed. People respond to, as I said, cultural mores and institutions. Economic considerations can drive changes in behavior, duh. That's what I said. You disagreed.
I'm quite unsure what you mean by a person's "nature." Perhaps you could define it more clearly, apparently without reference to behavior?
Here are some other areas that demonstrate how people have always been the same: Morally, people have always thought it best to not be strictly selfish. The culture in which a person resides will go a long way in influencing their beliefs about who one should be unselfish towards and to what level, but selfishness has never been seen as a good trait. Along those same lines, people have always thought that one should look after their offspring and care for the future generations. Honesty has always been considered a virtue, and cheating another person is never so. These are the kind of things that underlie human nature and they are the things that simply do not change.
Apparently you don't read much anthropology.
Some (admittedly few) cultures glorify simple selfishness, consider lying a gift, exhibit wildly different child-rearing practices, have different mores regarding murder, etc. I have a hard time imagining any moral precept to which all cultures in all times assented. All had SOME moral precepts, had SOME idea of right and wrong, a sort of moral nature... but there is no central "human" moral compass on which all cultures can agree. Even supposedly basic instincts like truth-telling and kinship bonds are conditional based on cultures (evinced by the fact that some, admittedly very rare, cultures have extremely different moral and social conventions for those things).
Finally, my point was not that people's behavior would completely reverse. My point was that it would be different. You can't seriously be telling me that you think the moral behaviors of Medieval people are readily analogized to those of modern people. We just don't believe the same things, and our beliefs actually do have an influence on our actions. Now, maybe not enough to make a society of 90% adulterers become a society of 5% adulterers, but a shift from, say, 60% to 40% is significant and plausible, given that we have seen shifts of comparable size in just the last 60 years.