The Bible makes it clear that Christians are to be completely intolerant of sin.  Not only that, sin must be removed from a believer's life.  Any way you look at it, my friend's sister and her fiancée having sexual relations apart from marriage goes against the clear teaching of Scripture; therefore they're intentionally going against God!  Author David Kupelain put it this way:  "Scriptures as laid down by the Hebrew patriarchs and recorded in the Old Testament -- and as reiterated largely intact in the New Testament -- constitute the objective and immutable standard by which morality, sexual or otherwise, should be measured.  And they believe that sex, other than monogamous, heterosexual sex within the confines of holy matrimony, is immoral and sinful." [2] 

In regards to sexual sin, C. S. Lewis pulls no punches,

"Our warped natures, the devils who tempt us, and all the contemporary propaganda for lust, combine to make us feel that the desires we are resisting are so 'natural,' so 'healthy,' and so reasonable, that it is almost perverse and abnormal to resist them. Poster after poster, film after film, novel after novel, associate the idea of sexual indulgence with the ideas of health, normality, youth, frankness, and good humour. Now this association is a lie. Like all powerful lies, it is based on a truth ...that sex in itself (apart from the excesses and obsessions that have grown round it) is 'normal' and 'healthy,' and all the rest of it. The lie consists in the suggestion that any sexual act to which you are tempted at the moment is also healthy and normal.  Now this, on any conceivable view, and quite apart from Christianity, leads to impotence, disease, jealousies, lies, concealment, and everything that is the reverse of health, good humour, and frankness. For any happiness, even in this world, quite a lot of restraint is going to be necessary; so the claim made by every desire, when it is strong, to be healthy and reasonable, counts for nothing. Every sane and civilized man must have some set of principles by which he chooses to reject some of his desires and to permit others. One man does this on Christian principles, another on hygienic principles, another on sociological principles. The real conflict is not between Christianity and 'nature,' but between Christian principles and other principles in the control of 'nature,' for 'nature' (in the sense of natural desire) will have to be controlled anyway, unless you are going to ruin your whole life." [3]

There's no getting around it.  Sex without holy matrimony is a sin, so "quite a lot of restraint is going to be necessary."

Recently I was listening to a friend give a play-by-play of a football game he'd just watched.  The man could have given John Madden a run for his money!  He loves the sport and it shows.  His statistical knowledge of the game was nothing short of remarkable.  Here's the downside. The guy's a professed Christian and I happen to know that he couldn't find the Gospel of John in his Bible if his life depended on it.  Why, I ask, do Christians ignore the very words of Jesus?  Our Lord spoke plainly when he said, "Whoever serves me must follow me..."  (John 12:26 ) And, "If you love me, you will obey what I command." (John 14:15)  Again, "If anyone loves me he will obey my teaching" (John 14:23).  Jesus wasn't just talking to the Apostles. He's talking to all Christians!

Which brings me back to my biblically illiterate friends.  Jesus also said, "I am the true vine and my Father is the gardener.  He cuts off every branch in me that bears no fruit, while every branch that bears fruit he prunes so that it will be even more fruitful" (John 15:1-2).  For clarification, Jesus Christ is the vine and the fruitful branches are Christians who, through a personal relationship with Him, will bear much fruit.  The unfruitful branches are the people who make a superficial commitment and bear little or no fruit.  The Father will prune the vine and discard the fruitless branches.  He offers no second chances.

Those who professes Christ and behave like heathens could find themselves lopped off the vine! Not because God is intolerant.  On the contrary, God is long suffering, not willing that any should perish.  Those who are separated from the vine have a superficial commitment, not a genuine faith. 
Continued...