Depends on the context. Between to people married to each other it is definitely not a sin.
who said so?
It really isn't, but certain religious institutions have instilled that in the general public's brains. That's why you're using the religious buzzword "sin" to describe how it's wrong.
There are certain times that it is, but all sex is not a sin.
It isn't unless it meets a few criteria
sex isn't a sin!
no it isn't it is a gift given to us by God for the purpose of procreation, and pleasure