The rule is that nonrestrictive relative clauses are separated by a comma, while restrictive relative clauses are not. There is an additional "rule" that which is only used in non-restrictive clauses and that only used in restrictive clauses, which is probably the source of the rule you learned. But this rule is the same sort of nonsense as not separating infinitives and not ending sentences with a preposition, that is it is based on someones idea how the language should work rather than any observation how it does work and hence it does not match the intuitions of a native speaker.

In German the difference between a restrictive and a nonrestrictive clause is not defined the presence or absence of the comma and there appears to be no easy and straightforward rule. (Faustregel: Bestimmtes Bezugsnomen -> erläuternder Relativsatz, unbestimmtes Bezugsnomen -> einschränkender Relativsatz)

Compare:

• Sie streben ein Eherecht an, das eine hinreichende Abschreckung gegen Ehebruch darstellt. [restrictive]

• Unser Eherecht, das eine hinreichende Abschreckung gegen Ehebruch darstellt, findet seinen Ursprung in dem Bestreben... [nonrestrictive]

The quoted version says something about a marriage license, and explains that marriage licenses ensure punishments... . The corrected version (without commas) says something about a marriage license which ensures such punishment, but makes no general statements about marriage licenses.