Matthew 5: 31 And it hath been said, whosoever shall put away his wife, let him give her a bill of divorce. 32 But I say to you, that whosoever shall put away his wife, excepting for the cause of fornication, maketh her to commit adultery: and he that shall marry her that is put away, committeth adultery.
The ‘excepting for the cause of fornication’ indicates that the only way in which a man can ‘put away a wife’ is if there is no true sacramental marriage, in which case the two are simply living together in fornication. And thus, it would be a GOOD thing for a man to ‘put away a wife’ if they are, in fact, not validly married. This is, in fact, what is recommended for any persons in an invalid marriage: to ‘refrain’ from fornication with each other, as they are not ‘truly’ married.
Matthew 7: They say to him: Why then did Moses command to give a bill of divorce, and to put away? 8 He saith to them: Because Moses by reason of the hardness of your heart permitted you to put away your wives: but from the beginning it was not so. 9 And I say to you, that whosoever shall put away his wife, except it be for fornication, and shall marry another, committeth adultery: and he that shall marry her that is put away, committeth adultery. 10 His disciples say unto him: If the case of a man with his wife be so, it is not expedient to marry.
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“Except it be”… In the case of fornication, that is, of adultery, the wife may be put away: but even then the husband cannot marry another as long as the wife is living.
This also explains why the Hebrews divorced, under the ‘old’ law, but that Jesus under the new law was holding them to the original covenant God gave men and women in marriage.
Notice too that His disciples said that "it is not expedient to marry’. Now, if what Jesus had MEANT was, “you can divorce your partner if he or she commits adultery or had a history of fornication”, as many Protestants believe, it would NOT be ‘inexpedient’ to marry, as one could easily find the pretext to divorce, even comitting adultery oneself if needed. But if only those not validly married are free to separate from a marriage bond, it IS difficult. Marriage then becomes (as intended) sacred, sacramental, and ‘for keeps.’
Mark 10: 2 And the Pharisees coming to him asked him: Is it lawful for a man to put away his wife? tempting him. 3 But he answering, saith to them: What did Moses command you? 4 Who said: Moses permitted to write a bill of divorce, and to put her away. 5 To whom Jesus answering, said: Because of the hardness of your heart he wrote you that precept.
6 But from the beginning of the creation, God made them male and female. 7 For this cause a man shall leave his father and mother; and shall cleave to his wife. 8 And they two shall be in one flesh. Therefore now they are not two, but one flesh. 9 What therefore God hath joined together, let not man put asunder. 10 And in the house again his disciples asked him concerning the same thing.
11 And he saith to them: Whosoever shall put away his wife and marry another, committeth adultery against her. 12 And if the wife shall put away her husband, and be married to another, she committeth adultery.
In Mark we see that Jesus not only reiterates the same position as given in Matthew and Luke, He further references the creation and that marriage involves one man, one woman, becoming one flesh–a ‘new person’, so to speak.