Do you believe it is within God's will or outside of God's will for persons to be engaged in sexual activity when they are not married?
Without getting into a Clintonesque prevarication along the lines of “That depends on what ‘is’ is”, we have to understand what we’re referring to when we say “God’s will”.
As you know, I believe God experiences time transcendently, that to Him everything is always happening Right Now simultaneously and forever.
We humans have a limited perception of time; for us free will -- the ability to shape the future by choices and actions we take now -- exists. To God, those choices and their consequences are always happening simultaneously and, to Him, are immutable.
Humans cannot be moral without free will. Breathing oxygen is not a moral act, it is one we do not so much out of necessity but of unavoidability. We are designed to breathe oxygen; we can’t not breathe oxygen.
There are some things God has ordained: Christ’s birth, ministry, atoning death, and resurrection, for instance. Those were definitely God’s will.
But there are other things that God does not actively will but will permit due to the culture of a particular time, place, and people.
God has only communicated directly with the human race twice, all other times He has inspired prophets and poets and philosophers and preachers to express His truth in their own words, in a manner that the people of their particular time and place would understand.
The two times He communicated directly with humanity was when He inscribed the Decalogue with His finger and gave them to Moses (whom promptly smashed them in a fit of pique, but more on that in a bit) and when He sent His only son Jesus to minister and teach.
In the Decalogue, God tells what the Israelites (and presumably, humanity at large) must do to live in a covenant relationship with Him, and among those ten things are “Thou shalt not commit adultery”.
Adultery is a betrayal of a marriage vow, it is not necessarily sexual relations outside of marriage. God permitted Moses, David, and Solomon multiple wives; their sexual relationships with those individual women were not adulterous even though they were married to other women (or had concubines, in Solomon’s case).
David’s relationship with Bathsheba started out in adultery, because they were betraying Uriah, her husband.
Jesus taught that it was God’s plan for one man and one woman to be married, to form a lifelong bond with one another. Jesus also said that ideally we should be so focused on our love of God that sexual desire paled in comparison, but if we could not quench that desire, it was better to marry than to go chasing after mere physical pleasure.
While I believe it is God’s will that we remain sexually inactive prior to marriage, I do not believe He considers all sexual activity prior to marriage to be sinful. A person who is engaging in sexual relations for selfish ulterior motives (prostitution, pornography, politics, etc.) is probably committing a sinful act. A couple of teenagers who get carried away at the drive-in may not be.
And without splitting the hairs too fine, what do we mean by marriage? If two people decide they will commit to one another and start co-habiting without benefit of clergy or civil law, that’s not sinful in and of itself; it’s certainly less sinful than celebrity marriages with the lifespan of a blowfly or relationships where one or both partners are cheating on the other.
 Although many times these prophetic utterances carry more than one level of meaning, and future generations come to learn that second meaning when they see it fulfilled.
 I think if David had been patient and waited a few months, Uriah would have been killed anyway in combat, thus freeing Bathsheba to marry him and sparing them the loss of their first son, the one born out of adultery before Solomon.
 And I know there are people who feel that premarital sex is “pre-adultery” against their eventual spouse and as such a sin. I think that’s a wonderful, sweet, romantic, noble thought and if they truly believe that they should hold to it, but it is not their place to judge the beliefs or regulate the actions of others.
 And I also believe that it is possible to commit adultery without actually engaging in a physical act, not merely in the way Christ taught about lusting in the heart, but in carrying on flirtations, correspondence, etc., with the intent of betraying a spouse. Thanks to the Internet, it’s now possible to have a virtual affair with a person halfway around the world and never see him or her face to face -- or for that matter, even know if it’s a him or a her…