Nothing is a cause for divorce save fornication. It does not matter how difficult it may be, it does not matter what the stress or the strain, or whatever can be said about the incompatibility of temperament. Nothing is to dissolve this indissoluble bond save this one thing… It is this question of the “one flesh” again; and the person who is guilty of adultery has broken the bond and has become united to another. The link has gone, the one flesh no longer obtains, and therefore divorce is legitimate. Let me emphasize again, it is not a commandment. But it is a ground for divorce, and a man who finds himself in that position is entitled to divorce his wife, and the wife is entitled to divorce the husband.
Martyn Lloyd-Jones