Sagittarius in the Navamsa places Jupiter's mutable-fire expansiveness over the marriage house, and classical readings treat this as one of the more auspicious placements for the D9 — Jupiter is the great benefic, and its presence here is generally read as bringing good fortune, higher learning, or a genuinely principled quality to the union.
A spouse indicated by this placement tends to be optimistic, philosophically or religiously inclined, and often connected to travel, foreign residence, or higher education — themes that frequently show up literally in the marriage's circumstances, not only in the partner's temperament. The relationship itself is often marked by a shared sense of larger meaning or purpose.
Sagittarius's own restlessness is the caution: a native or spouse with a strong need for freedom and expansion can chafe against a marriage's practical constraints, and Jupiter's characteristic bluntness can read as tactlessness in close quarters. The corrective is treating the relationship itself as one of the expansive pursuits worth committing to, not an obstacle to them.