I think part of it may be setting a "limit" which is allowed to be transgressed in order to satisfy the human desire for transgression of taboos.

If you tell your children that swearing or drinking is "bad", then when they get older they will drink and swear any way, but because they know it is "bad" they will regulate their relation to it -the idea being moderation. Apply this principle socially, and you've essentially created a "space" for this natural outlet which is removed from "public space" where it doesn't belong and would have negative social effects. It's the same reason you don't defecate in public. Everyone defecates, but there's a time and place for it.

If this limit was not set, then swearing and drinking would be considered "normal", so then they would go looking for other things which are "bad"; things which are probably much worse any way. If everyone did this, it would be a social disaster.

The same thing applies to sex, but sex is more complicated because sex can result in pregnancies, and uncontrolled pregnancies can destroy families, and that never leads to healthy and stable social relations when it becomes a widespread problem. The point of the whole "sex is bad" thing is again, to establish a taboo which will be crossed only when they are able to moderate their relation to it so that it doesn't lead to excess.

Taboos of some sort are necessary because when "everything goes" or "all is permissible" is taken to its logical conclusion, you end up with a real circus.

TL;DR: Humans have a need for transgressing taboos, but this need cannot be satisfied unless there are limits to be crossed in the first place. Without these limits, excessive or bad behavior become normal and this leads to negative consequences.