What are the traditional defenses to a divorce on fault grounds?

Actually, there still are defenses to divorce, in those states that recognize fault divorce. Collusion is a classic defense to divorce and the other C defenses: condonation and connivance. Provocation is another defense to divorce.

Before no-fault, fault was required for a divorce. Some couples may have wanted to divorce, but neither of them had committed any of the traditional faults: adultery, cruelty, desertion, imprisonment. In order to obtain the divorce, they colluded or agreed to say that one of them had committed a fault that would give rise to divorce. Usually this was a claim of adultery by the husband. The couple essentially was trying to defraud the court. If before the divorce one spouse wanted to back out of the divorce, he or she could claim collusion and reveal the agreement.

The movie The Gay Divorcee, starring Fred Astaire and Ginger Rogers, comically depicts a divorce involving collusion.

Condonation is approval of certain behavior. If someone does not object to something, one could be said to have condoned it. For example, if a wife sues her husband for divorce on the grounds that he committed adultery, he can raise the defense that she condoned it, or knew about it and did not object to it. She would be unable to obtain the divorce in that case.

Connivance is a secret plan to achieve an evil or illegal end. Let’s say, for example, that a wife wished to get a divorce from her husband. If she invites someone to stay at the couple’s house while she is absent, and that someone seduces the husband into an affair, the husband could later claim that the wife connived or set him to commit the adultery in order to get a divorce.

Provocation is behavior that incites a response. For example, if the wife sues the husband for divorce, claiming desertion, the husband can claim as a defense that she provoked his absence or desertion. She would be unable to profit by her bad acts.

Needless to say, provocation is not an adequate defense to a claim of physical or mental cruelty. "She was asking for it" will not fly with any judge.