How is doing something nature intends us to no (Programs us to do) sinful?
Did your God make us in a way that we would desire sex regularly, but deny us this privileged unless we dress in silly garb, say silly words, while other silly people stare at us.
I've often read the argument that what's "natural" surely can't be bad for us and we should be able to indulge in it as freely as we want.
And then I saw a counterpoint that said, "Poison ivy is 'natural' too... So why is it that you don't go around freely rolling around in it freely?"
God makes a lot of things that seem "natural" to us, but puts guidelines around them due to His Sovereignty, and for our own good. Nearly everything comes with guidelines.
Eating is "natural" too, but if you don't follow certain guidelines for it, it can surely kill you.
It all boils down to whether or not one will obey God, and in the end, He's the only one you have to answer to, not us. Believe me, I went to Christian schools from ages 5-18. NO ONE tries to argue their way around God's rules in order to justify unmarried sex more than a bunch of hormone-laden, rebellious teenagers at a Christian school. I've heard the case for "fornication justification" argued every which way there is.
But after all is said and done, God will be the only one you'll have to worry about justifying your answers to.
P.S. If sex outside of two people married to each other is "perfectly natural" and ok, are you then perfectly comfortable with the idea of someone having sex with your future wife? Because if you open the door to sex outside of marriage, you're also giving anyone else out there permission to sleep with your wife.
I knew someone once who truly believed that only someone who was married and having sex with someone other than their marriage partner was guilty of sexual sin... and therefore, if they chose to sleep with a married person, they, being unmarried, were not sinning, but the married person they slept with was.
I personally believe that this is one of the reasons God put the boundaries on sex that He has--because otherwise, anyone, anytime, and under any circumstance, is game. Including the person you're married to. (I realize people already override this as it is, but this is not what God originally intended.)