Regarding all the Christian responses that basically say "Because God says so" - I must point out that it appears the BIBLE says so - why are you so certain that GOD necessarily says so? Many, if not all, of the things written in the Bible either aren't God's word at all or have been potentially manipulated by others. It is far more likely that prohibitions against sex were generated by early Christian founders, and not Christ or God themselves. Many liberal and skeptical Christians do not take the entirety of the Bible to be the infallible word of God, and those of us who are reasonable enough to concede - Christian or not - that the Bible contains errors, contradictions, and fabrications, and is therefore at best only partially God's word, would encourage you to consider not abstaining from fornication or anything else prohibited merely because the Bible indicates that "God says so" .Now, as far as something like murder, theft, lying for personal benefit, and so forth, it seems obvious that such behavior is unethical within the Christian framework. Good *reasons* - that is, justified reasons that point to real-world consequences and such - are necessary for an action to be considered unethical by any rational code of ethics. Christians are capable of maintaining a more rational code of ethics only when they abandon the refutable conservative position that the bible is infallible.
My contention is that "God says so" is an insufficient reason for or against any particular action, especially if one is not absolutely certain that God says so. I don't consider words in the Bible to be a source for absolute certainty about anything (much less confidence). Neither is it sufficient reason to abstain from any behavior. Rational ethics are derived from secular sources - we recognize murder, theft, and other violations against one another as "wrong" for many reasons - the Bible saying so is not one of them, or at best isn't their source or isn't the only source. People in non-Christian societies and pre-Christian societies were ethical without Christianity, and non-Christian societies will continue to contain many of what are probably essential ethics - prohibitions against murder, theft, etc. "Do not have sex unless you perform an arbitrary religious ceremony that doesn't really mean anything and which usually ends up screwing everybody over and making them unhappy and ending in divorce, emotionally displaced and distraught children, and financially injustice" isn't one of them. Marriage today is a crock, anyway, and the Bible, unchanging as it is, is useless for addressing contemporary marriage issues.