Why sex in public is generally disapproved of by society

Society's sexual taboos today (unlike a couple centuries ago) are mostly about not embarassing anyone by showing off that you have more or better sex than they do. This is why following things are frowned upon:

• Looking at the penis of the guy in the urinal next to yours -- his could be bigger or smaller than yours. Nobody should know about this. Could be embarassing.
• Having sex in public -- there are many people who don't have sex at all.
• Being overly seductive in public -- it's embarassing for those people who aren't all that sexy.
• ...

I'm sure you could continue the list. The point is: Much like beauty, our society does not consider sexual activity or sexual attractivity a merit that is earned. And basically everything is frowned upon which cannot be earned in socially acceptable ways. After all, people even frown upon intelligence, sometimes.

An evolutionary reason for sex in public being worse than the alternative is not hard to find – you are exposed to attack while having sex, and therefore try to do it where you can’t “get caught”. While this is not the only explanation, it is sufficient.

Other reasons might be on a direct evolutionary level such as the one above, or might be more complex – our mind evolved in ways that made it susceptible to social pressure, and such pressure is not necessarily evolutionarily conducive – read more about memes if you want to know about that.

As for sex being an intimate act one does not do in public, etc., that is on a totally different level of explanation than the one sought by kaatunut, though it may be just as true.

Let's clear up some things here:

1. This is a big question. I am an anthropology major, I've heard many things about many cultures, and most cultures do not like people having sex in public. I doubt that it would be possible to limit the discussion to just 'our society'--it may be best explained through basic human nature.

2. In case you haven't figured it out from #1, this is not a European invention. People all over the world have this same taboo.

3. But, the taboo is expressed in different ways. For example, in many cultures people used to (and still do) live in small houses. For example, everybody in one room. Sex was carried out with-in arms reach of everyone else in the household. This was not loud sex, and the other people (most often, the children) weren't interested in (or at least politely ignored) what was going on. Sex out in the middle of the village, however, was right out.

4. Evolution may have had something to do with the forming of this taboo, but probably not because you are more "exposed to attack". When you are exposed to danger, your best bet for survival is to be surrounded by friends, not off by yourself. If exposure to danger was a large factor, we would more likely have a taboo against having sex without other people around.

5. This is a much wider taboo than 'no masturbating' or 'no sex out of wedlock'. Those are taboos existing only in a few cultures (well, hundreds of cultures, actually. But not all that many, overall). So don't blindly compare them.

I'm going to cut it out with numbering the paragraphs here, because I'm starting on the pure speculation bit here.

Probably the reason humans don't have sex in public, and feel that it is wrong for other people to have sex in public, is because, as has already been stated, sex is a private thing. For humans sex is social, and takes place on the dyadic level.