A man shall leave his father and mother and be united with his wife, and the two will become one flesh. So they are no longer two but one
Mark 10:7-8

Have you ever sat in a marriage service and wondered, "When are those two actually married?" At what point are they man and wife together? After there vows is it? After the exchange of rings? Or is it when the priest says "I therefore proclaim that they are husband and wife" and then, joining their hands says "That which God has joined let not man divider." Is it then, or maybe later when the marriage is consummated. An unconsummated marriage may be declared null and void

Let not man divide
At that point, I suspect, most couples have little idea of dividing. Though I do recall one bride-to-be on BBC Daytime Live saying something like "In five or ten years time we'll both have changed. We'll be different people, and feel differently about each other. That'll be the time to split up and find fresh partners." I wonder how many they spoke for, and maybe voiced an unspoken agenda? Statistics show that one in three marriages ends in divorce. Divorce has become the soft option.
  The scripture from Mark brings us face-to-face with the issue. What did Jesus teach? He was asked by the Pharisees, who were laying a trap for him, Is it lawful for a man to divorce his wife? If he said "Yes" (which they taught) they could accuse him of being at odds with John the Baptist (whom the people respected). If he said "No" they could give him over into Herod's hands. They were trying to get Jesus in the same type of Catch 22 as they later tried over the issue of paying taxes.
  Jesus, in his reply, went back to the Scriptures as first principle. In this case his answer opened up the wider issue of sex and marriage. And indeed it's only as we understand God's intention in sex and marriage that we can understand the truly Christian position on divorce. So Jesus takes them back to Genesis and Creation, to those words we heard quoted by Jesus. Before I go further let me say this:

Much of the reading from Genesis 2 had to do with Creation. We read in verse 7:
  The Lord God formed man from the dust of the ground and breathed into his
  nostrils the breath of life and man became a living being.
Then in verse 22:
  The Lord God made a woman from a rib he had taken out of the man
and then in verse 23 Adam says of Eve:
  she was taken out of man.
Do we really believe that today? Don't we think we know better? Don't we know man evolved, and evolved as male and female from the apes? How many have read Desmond Morris's best-sellers The Human Zoo and The Naked Ape ?
  In that atmosphere little wonder that our teaching on sex, marriage and divorce has lost its roots! We think we are animals! Morris has actually written another book: Intimate Behaviour on a scientific comparison of human sexual behaviour and the ape's.
  But, do we really know? Is evolution truth? Is it not just a theory? Might it not just be that God did create humankind, and woman from man (which is not just a chauvinistic remark!) It's only as we return to the truth of creation teaching that we can have a true understanding of the mind and will of God on sex, divorce and marriage.

That brings me to a second point about sex. (by which of course I mean sexual union)
  The man shall be united to his wife
The word which many modern translations have as 'united' (and the Authorised Version as 'cleave') is the Greek word proskolla'o: it's a strong word and its root is in kolla which is the word for 'glue' So the joining together of man and woman in sex is no casual affair. It commits them together. It cements them together. There's an intensity in the human sex act which is not just biological. We call is 'making love'.

So to a third point. 'Making love'. Sex is creative and not just procreative.
  So the two will become one flesh. They are no longer two but one.
So the act of sex, and this is God's intention, makes something new; an integral part of his creation. That's why masturbation misses the mark, for sex is not just about gratification of a physical urge, but bodily union with another person. That is why 'casual' sex can never be that. And, of course, it shouldn't need saying: that is why homosexuality is not allowed. It is an unnatural act.
  There is a permanency in God's intention in sex. They are no longer two but one So if you go off and find another partner that's a denial of the union that has already taken place. God's will is just this: Sex is to be within a lifelong faithful marriage. For-
  A man shall leave his father and mother and be united to his wife.

That's the point I've been heading up to. All I've said about sex is within this final aspect. That its place is in a permanent and faithful marriage relationship. For it's only within marriage that this new 'one flesh', this new unity can grow and develop. It's this which gives the special intimacy to marriage; and intimacy which depends on, but goes beyond the sexual union It's marriage which is in mind in that strong word for bonding of man and woman. The force which drives them together can only be safely confined within a permanent marital union. And it is this union which makes sex good. Every act of sex out of marriage is wrong and sinful. For the simple reason that it is against the will of God in creation. Every other command of God has to do with the healing of the result of sin: the command for man and woman to be one in marriage is absolute, sine qua non ; and when fulfilled has a special beauty and purity just because it comes from man's unfallen state.

So, what do I need to say about divorce? It is allowed Jesus said because your hearts are hard ; the real sense implying dryness. If we turn to the parallel account in Matthew Jesus says more explicitly that divorce is allowed on the ground of adultery which of course is an attack on the very base of marriage. Must we not see adultery as a murderous act; murderous of that new creation of the marriage union? The command against adultery comes next after murder.

One final word. Sexual sin and unfaithfulness have been taught as the most serious sin. (Actually sins such as pride and hypocrisy are more grievous in God's eyes) Yet sexual sin can leave such a massive legacy of guilt. So let's not forget: Jesus died for every sin and his blood will cleanse from every sin. Jesus has the power to take away all sin and deliver from all bondage.