Robust is my favorite adjective for the #1 quality for a marriage. Below are similar descriptors of the same quality.

— Resilient is comparable to Robust, but it implies adaptability. Marriage is based on immutable principles cast on stone, but they should not be rigid. Pehraps Resilient is a good adjective after all.

— Strong is very close to Robust, less close to Stable. Strong implies invasions, breaking pressures etc, whereas Robust implies setbacks,  failures, breaks, damages/injuries/casulties, which are common challenges and actually test and define the strength of a marriage.

— Stable is my #2 favorite adjective. Robust has a richer meaning.