How can there be an option for remarriage if old covenant law required adulterers to be put to death?
Old covenant law required that adultery be punished with death (Leviticus 20:15; Deuteronomy 22:22–24; John 8:1–11. There would be no option for remarriage if an adulterous affair was in view. Possibly the law stipulated the greatest punishment that a transgression required, rather than the only one (King David). In other instances of sexual crimes, a monetary fine was imposed (Deuteronomy 22:13–30). Given the fact that the Israelites were not free to enforce the death penalty in the time when Jesus ministered on earth (see John 18:31), a monetary fine was even more likely.
32 But I say to you that everyone who divorces his wife, except on the ground of sexual immorality, makes her commit adultery, and whoever marries a divorced woman commits adultery.