If that's true then there's no moral binding for any of us. Conscience is an illusion. Thankfully, neither is the case, and your statement is completely ridiculous.
However, the thought that sex should only be for reproduction is also ridiculous. It can indeed be for pleasure, but the Bible says it's only for married people to enjoy. Now if you don't believe that, that's your prerogative. It's not Christians' responsibility to enforce the moral standard, merely to make it known. God can do His own enforcing.
If only more believers would heed that simple and elegant piece of advice.