But I say unto you, That whosoever shall put away his wife, saving for the cause of fornication, causeth her to commit adultery: and whosoever shall marry her that is divorced committeth adultery.
Is Matthew (Jesus) saying that divorce is acceptable in the case of adultery, or is he saying that premarital sex, while betrothed, is the only legitimate reason for a divorce?
The King James version and the original Greek appear to use the word "fornication," meaning premarital sex exclusively but the meaning of this word seems to have changed over time to include adultery.
That change in the meaning of the word "fornication" further begs the question: Did it's definition change to fit the (Protestant) church's desire to allow divorce?
I will add this:
The Torah differentiates between two categories of husband: an arus, or betrothed, and a baal, or full husband. Under Torah law, marriage consists of two distinct stages. First comes the betrothal (erusin), by which the bride becomes "forbidden to the rest of the world." 3 From this point on, for another man to have relations with her is tantamount to adultery, and to dissolve the betrothal requires a get (writ of divorce), as for a full-fledged marriage. The betrothal, however, only establishes the prohibitive side of marriage (the exclusion of all other men from the relationship), but not the substance of the relationship itself—the two still cannot live together as man and wife. This is achieved through the second stage of marriage, the nissu'in, which renders man and wife "one flesh." 4 In Biblical and Talmudic times, the eirusin and the nissu'in were held on two separate occasions, so that for a certain period of time (usually a year) the bride and groom were bound by the prohibitions of marriage but had not yet begun their actual life together. In this period, the groom is called an arus; following the nissu'in, he assumes the status of baal. (source)