http://christianity.stackexchange.com/questions/11518/why-is-sex-between-two-consenting-adults-sin

My understanding is that murder is a sin because I don't have rights on another person's life in the same way that another person doesn't have rights over my life. Likewise stealing would be a sin because I don't have rights over another person's property nor would I like anybody else to take what is mine. On the other hand smiling would not be a sin because I don't encroach on another person's right when smiling and they may actually like being smiled upon.
In the Old Testament David (considered a prophet) did not sin when he engaged with concubines and slave girls who were not legally wives. If adultery is defined as having sex with another man's wife, David has not broken the law. Or did he? However today if two (unmarried) consenting adults have sex, that's adultery by Biblical standards. From my understanding its encroaching on another person's right that seems to define sin. Here I don't see an encroachment of the rights of any person.
If a married person is involved in extra marital sex he sins, because he has broken his marital vows. Why is the sexual act a sin when two people have fun by mutual consent and not encroaching on each others freedom?

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Probably not the "sex" that is the sin, but the "lust" that fuels/ creates the want for sex. Christianity, like many other religions, has a core of self-denial. Jesus says in Matthew 16:24, "If anyone wishes to come after Me, let him deny himself, and take up his cross, and follow me”

http://jwvictims.org/2015/04/02/jehovahs-witnesses-believe-that-minors-can-consent-to-sex-with-adults/
Let me first of all say, I am well aware of the fact that minors under the age of 18 know all about sex, and many of them may be sexually active long before they are legally an adult. I don’t live in a fantasy world where I think sex is a big mystery to teenagers. That being said, let’s break this statement down a bit so we can understand the gross obscenity of these words, and how easily it puts children in danger as well as allows child rapists to freely roam the local Kingdom Halls of Jehovah’s Witnesses. How Young Is Too Young?
First note that they talk about a minor “approaching adulthood.” What does that mean? How young exactly, 17? 16? 15? Since an 18-year-old adult would then be a “few years older,” it’s certainly possible.
This may seem like a petty point, but it’s vitally important that non-JWs and rank-and-file members consider its importance. Since the governing body didn’t state specific ages or what was meant by a minor “approaching adulthood,” this gives local bodies of elders great leeway in determining this for themselves. They are then free to decide on their own that a 15-year-old or someone even younger had “consensual” sex with an adult, and a charge of pedophilia or child rape is dismissed in their minds. To them, a 19- or 20-year-old having sex with a 15-year-old is just sex between two consenting parties, not statutory rape. Lest you think I’m exaggerating this point, note this post, where the situation of a man going into the bedroom of his stepdaughter and fondling her breasts while she slept was referred to as a “minor uncleanness.” The real problem, according to the instructions given in this particular case, was that he touched the breast of a female to whom he was not married, not that he molested a child, and his sleeping stepdaughter at that. Never once is the word “molestation” or “pedophile” used in that case.
Yet again, Jehovah’s Witnesses show that they have no understanding of the horrific nature of child rape and molestation. Their only concern is if two people are married, not if one is a child who has been abused or taken advantage of, which leads to the next point.
Minors Consenting is an Oxymoron
Consider also that a minor cannot consent to sex with an adult, especially an adult who is “a few years older” than them. Again, I understand that teenagers know all about sex, but teenagers are also impressionable, naive, easily coerced, lack understanding of what is best for them, and at times need protection from themselves and their own impulses. It is imperative that adults understand this, and one who is a “few years older” than that teen is especially culpable.
This is exactly why the law backs me up on this point; even if a minor under the age of consent says that they want sex and seem ready for sex, it is still child rape to engage in sex with them. A minor is not allowed to make that decision for themselves, not legally or morally, for all the reasons I state above.
The fact that teens are not mentally and emotionally ready for certain responsibilities and activities, even if they seem physically ready, is exactly why we have particular laws in place barring them from those activities. For instance, in the U.S. you need to be 16 to get your driver’s license simply because it takes some maturity to keep yourself safe when behind the wheel. Whether or not you’re tall enough to reach the pedals is not the point; you need to have some seriousness of mind to understand that driving is not just fun but also dangerous. If an adult gave the keys to their car to someone underage, they would be in trouble legally and no doubt many people would question their judgment and character, no matter how much that child begged and pleaded to take the car. As an adult, it’s his or her responsibility to behave as the adult and set needed boundaries.
Calling the police on an adult who allows a child to drive is a good step to take, and I’m sure people can understand why a person should do that, for the protection of the child. Why then would Jehovah’s Witnesses fail to apply this same thinking to an adult having sex with a minor? This brings me to the next point.
Ignoring the Law as Well as Morals
The morality of just dismissing an adult having sex with a minor as being “consensual” is bad enough, but note that the law doesn’t allow for this reasoning either. If an adult has sex with a minor, it’s called statutory rape; it doesn’t matter if the minor said he or she wanted sex or seemed ready for sex, and it doesn’t matter if the minor is “approaching adulthood,” whatever that means. The law gives no leeway for a minor who is not yet at the age of consent to have sex with an adult. Despite that, there is no instruction for elders to call the police and report this crime if it’s becomes known to them. As I bring out in the Pedophilia category of this site, JWs see this act as a “sin” and not a crime; it’s “sinful” when the two are not married, but they feel no obligation to involve the law in these matters.
This brings me back to my original point. By not involving law enforcement, Jehovah’s Witnesses may very well be complicit in the crime of statutory rape. I don’t know when the law requires an elder to report this situation, but why would you report an adult having sex with a minor only when legally required? This person committed a very serious crime and if they face no consequences, what stops them from doing it again and again? Why should they be allowed to get away with breaking the law of the land, especially in such a serious matter? Determining Consent Another important and disturbing question is who exactly determines if the minor gave this so-called consent, if there is any question? Would it be the adult who had sex with that minor? Would the elders make this determination?
This too is not a point to gloss over; as I bring out in this post, elders actually have the power to determine if an adult woman who claims to have been raped was actually raped, based on some flimsy instructions regarding her “mental disposition,” the circumstances around the incident, and if she delayed reporting it. If elders believe they can and should sit across from an adult woman and tell her that no, she wasn’t raped but she did actually consent to being sexually violated, what would stop them from sitting across from a teenager and doing the same?
By assuming that minors can consent to sex with an adult, a dangerous atmosphere is created in Kingdom Halls. An adult can easily groom teenagers “approaching adulthood,” have sex with them, and face no other consequences than a meeting with elders who chastise and counsel them about “fornication.” Meanwhile, teens are coerced into relationships and sexual scenarios for which they’re not ready and which may affect them for the rest of their lives. Because of facing counsel and other punishment including shunning for “fornication,” they may also agree to marry someone at their young age, and then they become trapped in a relationship that is not healthy or fulfilling. As with most fundamentalism, high-controlling religions, divorce is not an easy option or way out for them, and of course having children at a young age can also contribute to a young person, man or woman, feeling and being trapped in many ways.
The law does not support this statement made by Jehovah’s Witnesses, and neither do good morals and character. The protection of children and teenagers from any type of sexual exploitation should be of utmost importance to adults in any setting, but this just isn’t the case with Jehovah’s Witnesses, as this letter proves. It also betrays the statement made by governing body member Stephen Lett on the TV channel for Jehovah’s Witnesses, where he insisted that the organization protects children from pedophiles (see this post). Not only do they fail miserably to do this, but they openly state their beliefs that a minor may very well have “consented” to having sex with an adult. The victims of the sick persons that prey on others in their religion are dismissed and perhaps even blamed for their situation. Shameful.