Exodus 21:7-11
Deuteronomy 24 uncleanness = sexual immorality (24:1)
Matthew 5:31-32; 19:6, 9
Mark 10:2-12
1 Corinthians 7
When a man marries a woman, he makes a vow to care and provide for her and all her needs. By not fulfilling those needs, he is effectively abandoning her, and since those needs include sexual fulfillment, that's what makes it sexual immorality.
Paul said:
Let the husband render to his wife the affection due her, and likewise also the wife to her husband.The wife does not have authority over her own body, but the husband does. And likewise the husband does not have authority over his own body, but the wife does.Do not deprive one another except with consent for a time, that you may give yourselves to fasting and prayer; and come together again so that Satan does not tempt you because of your lack of self-control. - 1 Corinthians 7:3-5
http://www.biblegateway.com/passage/?search=1Corinthians7:3-5&version=NKJV
In other words, a husband's body belongs to his wife, and a wife's body belongs to her husband.
Paul then says:
Now to the married I command, yet not I but the Lord: A wife is not to depart from her husband.But even if she does depart, let her remain unmarried or be reconciled to her husband. And a husband is not to divorce his wife. - 1 Corinthians 7:10-11
http://www.biblegateway.com/passage/?search=1Corinthians7:10-11&version=NKJV
When you teach someone to drive, you don't name the exceptions every time you give the rule.
Jesus said:
“Furthermore it has been said, ‘Whoever divorces his wife, let him give her a certificate of divorce.’But I say to you that whoever divorces his wife for any reason except sexual immorality causes her to commit adultery; and whoever marries a woman who is divorced commits adultery. - Matthew 5:31-32
http://www.biblegateway.com/passage/?search=Matthew5:31-32&version=NKJV
And in doing so, he stated the rule, AND the exception.
Which Paul later confirmed and agreed with in 1 Corinthians 7, and in doing so, he even gave en example of the exception, which would be legitimate grounds for divorce.