Sexually transmitted diseases are essentially bacterium, viral or some pathogen that has found a way to survive by infecting a host and recolonizing in other individuals through physical contact. The results of these infections in our physical and mental being as well as our social evolution as a society have been wide ranging.
Your question is three part and wide ranging. In this format I can only swipe the tip of the iceberg.
In the past it can be argued that most of our ideas of social norms and mores as well as the concept of monogamy evolve from the need to limit sexual contact. The same as certain dietary laws in religious practices prevent the spread of certain disease. There are many instances through out history of individuals who had succumbed to the madness of Shypillis and the actions of these individuals including murder, tyranny and war.
In modern times and we learned to treat these diseases and preventive measures for transmitting these diseases it can be easily argued that that may be one of the leading causes of the laxation of our morals as a global society. These changes can be observed on a macro scale in American culture from the changes in attitude from the early 1900 to the present. When there were no real cures for sexually transmitted disease thru the advent of penicillin, which could treat and cure many of the bacterial infections. Any of the baby boomer generations well remember the films in school warning us of "the clap". Again, look at the changes in social custom with the advent of AIDS, there was an overall decrease in sexual promiscuity. In the mid 80's it was common for people in the dating science to get regular tested and only engage themselves sexually with other individuals who could prove they had been recently tested. With the advent of drug therapy for AIDS there seems to be an overall decline in the concern over sexual contact.
Aside from the social issues there are the economic issues of sexually transmitted diseases. Look at the billions of dollars that have been spent and continue to be spent some good, some bad, some indifferent. Additionally, there is the mental affects of living with sexually transmitted disease. The affects on the overall quality of life and the effects that any given individual suffering from that mental anguish would have on the lives of the people around him. As well as the secondary individuals affected.
The future as always waits to be seen. This becomes the product of speculation. As AIDS ravaged population are decimated and the children of theses areas mature into the adults of the future in many third world areas what will be the overall cost to the world? What civil wars could evolve because of this? If the aids virus evolves to a more resistant strain what will be the overall impact on society? How many billions of dollars will we spend? How much of the world resources will be allocated? What is the delay in finding cures for other diseases because so many of our research minds are working on curing the disease at hand or the next disease to evolve?
Could eventually we return to a more monogamous society and build stronger families and closer communities?
There have been recent studies centering on DNA and the human genome project that suggest that there should be X amount of genetic diversity as compared to genetic diversity that currently exist. One of the theories that is being considered is that ever so often there is a severe reduction in human population. A natural thinning so to speak. This could be the result of genetic resistance to certain disease. Could this be sexually transmitted?