'Condoms' in Society, Not 'Knowledge'

Though my interests lie mainly with political economy, I do have those nerdy economic thoughts popping into my head from time to time. Now, I'm sure research has been done on what I'm about to propose, but either way, I know of no research.

Let's assume that condoms encourage more sex in society, because the use of condoms decreases the odds that one will knock somebody up or come down with the bug, which people consider bad, and generally seek to avoid these conditions. This may or may not be an accurate assumption, for one could counter that people are going to have sex just as much, with or without protection. True as that might be, it is an empirical question that I won't consider here.

Now, because people have more sex because of the existence of condoms, can we a priori say that societies using condoms will have lower incidences of STD contractions and unwanted pregnancies?

I think not.

Consider that a) condoms break, b) condoms possess a small margin of error without breaking, i.e., they don't prevent STDs and unwanted pregnancies sometimes, albeit a very small percentage of the time.

It comes down to an empirical question about the society at hand. Do the members of a particular society have so much more extra sex as to make the 'small percentage of the time' quite large in absolute numbers? That is, does the presence of condoms actually increase the frequency of STD contractions and unwanted pregnancies?

There are idiosyncratic situations that may or may not be considered significant, such as the scenario of the guy who thought he had a condom in his wallet, but found out he was wrong when it came to go-time, and said, "Screw it."(pun intended) This certainly does happen, but does it happen frequently relative to being right about having a condom?

Taking a guess, I would say that the presence of condoms in society generally gets the job done, considering the population growth of 3rd vs. 1st world countries, blaze blaze. But this issue isn't why I pose the question.

Instead, I'm questioning automatic assumptions that people have, regardless of having any evidence to support what they believe. Indeed, I've never looked into the effects that condoms have on societies, but I've always carried the notion that they generally decrease the rate of unwanted pregnancies and disease.

This is kind of like how people automatically assume that the government can do what it sets out to do, with the right satraps. Or, that industrialization or Splenda is inherently pernicious to human health. These two, in my opinion, are the easy fallacies to catch. But what other assumptions do we carry that are totally false? Is this phenomenon prevalent?

Alright, OK, what I'm really asking is, What assumptions do I carry that are totally false?

This question has been asked in many different forms, and it is often what economic research is all about. I don't think there is any uniform answer. But it is food for thought.