National Survey of Family Growth indicates that, even in the last five years, rates of sexual activity have changed significantly; notably abstinence in the US has risen, while exact preferred sexual acts have also changed.
http://www.cdc.gov/nchs/nsfg.htm
Change is confirmed in data from National Survey of Sexual Behavior:
http://www.nationalsexstudy.indiana.edu/
QED.
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.
To look at the kind of sexual change you have indicated simply shows that people's behaviors change over time, it does not show that their nature changes. The sex studies are micro level changes in society and what you would need to demonstrate to show that people change is a macro level change, something that wouldn't revert back if the surrounding culture changed. Show me a time when most of the people in a society didn't have sex, that would be a proof.
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.
