I am going to attempt something that many consider impossible: defending the compatibility of sexual liberalism with Christianity.
First: I define sexual liberalism as the idea that sexual desires are normal and expressing and fulfilling them are not shameful as long as they do not harm anybody. The last clause is the most important: as long as they do not harm anybody. For example: casual sex between consenting adults is always okay, as long as both parties are honest with each other in regards to STIs, birth control, etc., since deception that might result in health issues or an unwanted pregnancy is definitely harmful. Keep this criteria in mind.
First we tackle the idea that sexual activity outside of marriage is sinful (sinful defined as action which is offensive to God). We must ask two questions to define right and wrong within the confines of Christianity: "Does it hurt anybody?" "Does it promote love?" (Love the virtue, not the emotion).
No sexual act between consenting adults hurts anybody, except perhaps through the envy of a third party (which itself can be regarded as a sin; this matter could be discussed at some length, but we will set it aside for irrelevance). The second question requires a bit more thought.
Sex (henceforth meaning any sexual act performed between one or more consenting adults) in its greatest form is the ultimate expression of a specific kind of love (called eros by the Greeks) between human beings; in its least a simple physical activity with the purpose of fulfilling a strictly physical need, with at least some measures between.
It is no secret that sex is good for you, both physically and emotionally, and from there we see that even sex in its lower forms does indeed promote love, whether through forging or strengthening connections between people, promoting individual well-being, or as a sort of "practice" for when you do find that person with whom you can have the greatest form of sex with. In defense of this third one; how many people wait until after marriage to have sex for the first time, only to tragically find out that they are sexually incompatible?
However, the frequent occurrence of biblical passages, in both testaments, that refer to the evils of fornication and sexual promiscuity must be addressed. Our first line of defense is familiar: those passages reflect the values of an ancient culture that viewed sexuality very differently than we do. Our scientific understanding of sex has progressed vastly in the thousands of years since, and that does account for something.
I am aware that this argument could seem like a cop-out: "You could say that for almost anything in the Bible. If nothing from those cultures is valuable, why be a Christian at all?"
The first part of the answer is that nobody becomes a Christian because they are attracted to traditional Christian views on sexuality (or, if they are, there is something very wrong with them). The stuff about sexuality is peripheral to the purpose of Christianity, which is personal transformation through social justice into a state of being closer to God.
That said, there is a negative aspect of sexuality: when it becomes problematic in one's life, or leads to separation from God. So both love and justice are to be practiced in all forms of sexuality: that nobody is exploited, used, or marginalized. This is easily applicable to specific social issues (prostitution would be fully legal in any truly Christian nation so as to avoid exploitation of sex workers) but harder to practice in one's personal life. Thus I urge that caution be used before engaging in any sexual activity, so any possible negatives are taken into consideration.
My final argument has to do with the origin of sexual liberation in the west. Sexual liberation is the opposite of sexual repression, a tool used for millennia with the express intent of oppressing women. Thus Christianity, as an inherently feminist religion, supports sexual liberalism in our time.
Examples abound. The bible forbids sex outside of marriage, but polygamy is permitted in much of the Old Testament. The allowance of men to have multiple wives reflects ancient Israel's attitudes about sexuality: men were allowed to have multiple sexual partners, permitted they financially supported them. The distaff counterpart would have been laughable to them; they viewed women as a bare step above property.
Similar attitudes prevail even today. The opposition of many conservative Christians to contraception is rooted in the idea that sex should be reserved only for procreation, an inherently androcentric view (supported by feminist theologian Patricia B. Jung; the argument is that since ejaculation and orgasm happen simultaneously in men, sexual fulfillment is equated subconsciously with procreation, which is not the case for most women).
Bottom line: sexual repression is rooted in the misogynistic cultural norms inherent in the ancient civilizations that wrote the Christian bible. When deciding our sexual ethics, it is important to look beyond the bible's limited scope to the thousands of years of cultural, social, and scientific developments we have had since, and reconcile that information with God's passion as known through Jesus. Does it cause harm? Does it promote love? We have seen the answers to these questions.
This is no exhortation to have as much casual sex as one can have, but rather a suasion for each of us as individuals to get in touch with our sexual identities and desires, and to seek fulfillment and expression in safe, positive ways, whatever they may be. It is also a rebuke against all those who would use the name of God to oppress others. What is right for you may not be for others, and it is important to have love and respect for each other's differences, even our sexual differences.