Why is it that only Jehovah's Witnesses.?

The original and true Christian Church, the one Church founded by Jesus Christ, to which He promised the fullness of truth, has always absolutely condemned sexual activity outside of marriage.

Your point is moot, since no JW is a true Christian, because all JW's worship an angel, instead of the real Jesus Christ, who is the eternally begotten, divine son of the living God.

No offense, but this seems like one of those fake TROLL questions because it is SOO over the top and generalizes about ALL Christians - something that non-theists on the R&S section tend to do a lot of.. so sorry if I find this question difficult to take seriously.

Anyways...........

<<Why is it that only Jehovah's Witnesses..Obey the Bible by not consenting sexual engage ment out of wedlock>>

There are roughly 2.0 billion Christians scattered around the World. You seem to know what they did or didn't do - which is frankly impossible. 

While I agree that abstaining from sexual relations before marriage is difficult, especially in the hyper-sexualized culture of 'westernized' countries. -- There are SOME couples who have followed God's teaching on this.

To say that only JHs do this is either propaganda OR a generalization.

<<There is not one single couple within our religion ...>>

OK, while I STILL doubt you know what EVERY couple in 'your religion' (whatever that is) has done or does --- This is STILL more probable then your original claim about ALL 2 billion Christians. Especially if you are a member of a smaller community church that has only like 200-300 members or something like that. It is a far more realistic statement (though still doubtful) than your original.

So sorry your religion has so much issues with pre-marital sex. Maybe God is calling you to start a youth ministry to change that??? = Ever think of that?

<...that can be approved of being a true Christian. >>

Human categories -- This whole thing of judging people as 'true' Christians verses what else? -- fake ones?

WHO gets to determine which individual people are really a 'true' Christian?

YOU?

God can rightly judge, but it is not OUR place to categorize or pigeon hold someone as 'true' or 'not true' 

"Do not judge, less thee be judged"  -- Be careful - get too caught up in judging who among your brethren are "true" and that same judgement can come back on you 

Jehovah's Witlesses not use Christian Bible, use arbitrarily altered version, thus most Christians hate them. They are so obnoxious most people hate them. People's sex lives are their own business. How can you claim to know such things? I sad that religions strive for totalitarian control of people. If someone tell me when I can have sex, I slap him. Most Japanese and Chinese hate Abrahamic religions, since they demonize our bodies and their functions.

Only one reason:

because you imagine it to be so.

Certainly: not because it is so.

A patently false statement that cannot be validated. 

If you are claiming that no JW has ever had sexual relations outside of marriage, I have serious doubts that you could ever prove that.

"Let both grow together until the harvest. At that time I will tell the harvesters: First collect the weeds and tie them in bundles to be burned; then gather the wheat and bring it into my barn.'" (Matthew 13:30).

Who knows when that one committing fornication will hear the Word of God even in their sins and be converted?

"For what knowest thou, O JW, whether thou shalt save the fornicator?" (I Corinthians 7:16) if you were preaching the Word of God? 

“What man of you, having a hundred sheep, if he has lost one of them, does not leave the ninety-nine in the open country, and go after the one that is lost, until he finds it? 

And when he has found it, he lays it on his shoulders, rejoicing. 

And when he comes home, he calls together his friends and his neighbors, saying to them, ‘Rejoice with me, for I have found my sheep that was lost.’ 

Just so, I tell you, there will be more joy in heaven over one sinner who repents than over one sinner is turned away from his wrongdoing, than for ninety-nine good men, who have no need of a change of heart" (Luke 15:4-7)!

STOP that "disfellowshipping" people when there's a possibility of the lost sheep returning to the fold and "Only GOD draws" (John 6:44) but if you've kicked him out and preaching the Word of God, there's no possibility of him hearing and believing (Romans 10:14-15)!

I'm sure that's not at all true. Many Christians who are not J.W's do not have sex outside of marriage. But the first thing with becoming a Christian is admitting your own sinfulness. Confessing our sins to God through faith in Jesus and the work He has done on our behalf. Many people realise there own sins and turn to Jesus for forgiveness, which can only come through Jesus alone. That's called repentance. But "if you think you have no sin, you are a liar and there is no truth in you." Jesus said if you think lustful thoughts in your heart about any one, then you have committed adultery with that person in your heart. So the first thing is to realise and confess your sins. Not to point to the sins of others, as that is self righteousness.

A Christian is a person who knows God and His salvation through faith in Jesus. A Christian whom God has saved by the work of Jesus on the cross and through His resurrection from the dead. Thereby proving His power over death and hell, for all who believe.

Mat 5:28

Let Allah be the judge