Synonyms for Decorous (same or very similar meaning)
WordNet sense 1 (according with custom or propriety):
becoming, comely, comme il faut, decent, seemlyWordNet sense 2 (marked by suitability or rightness or appropriateness):
properWordNet sense 3 (characterized by dignity and propriety):
sedate, staidFrom the ODE community, based on WordNetadd/edit