What does Mark 10:4 mean?

The Pharisees are using weasel-words: terms that can be easily bent to give a preferred conclusion. The Mosaic law which God gave the Israelites allowed a man to divorce his wife, but commanded that if he do so, he must give her a certificate (Deuteronomy 24:1–4). Without the certificate, or "get," the woman would be banished but still legally married and unable to reclaim her dowry. Even today, devout Jewish women cannot be declared officially divorced without a get or widowed without two witnesses who saw her husband die. This has led some soldiers to give their wives a provisional get when they leave for combat duties.

The passages the Pharisees are referring to don't encourage divorce. They talk about what justifies divorce, what constitutes divorce, and if a divorced couple can remarry each other. Divorce was allowed if the woman was found to be "indecent." "Indecent" is from the Hebrew root word 'ervah and means something shameful that has been exposed. In practice and intent, it means adultery. Divorce was final if the man gave the woman a get and sent her out from the household. They could remarry each other, but not if either one of them had married someone else in the meantime.

That law sounds harsh to modern ears, but it was designed to protect women in a time and place that did little to legally honor them. There was no avenue for a woman to divorce her husband, even for abuse, but—just like slavery in the Roman Empire of the New Testament—civic laws were not going to change the hearts of a culture. God's law prohibited men from sending away a wife, without her children or dowry, for spurious reasons like burning dinner. God made it clear that marriage is for life, and a woman is to be protected. Jewish men had the same hardened opinion of these restrictions as the disciples (Matthew 19:10) and altered the law. God made male and female (Mark 10:6), but He did not make the female to be disposable.