Egdon Heath definitely operates based on tradition and a good dose of expectations for certain social groups. In The Return of the Native we see people failing to performing their expected "roles" in this book, and we witness the ramifications of not playing by the rules and norms of such a traditional society: Mrs. Yeobright embarrasses herself and her family when she publicly refuses to support Thomasin's engagement; Eustacia is shunned and is even persecuted as a witch; Clym is seen as a disappointment for returning home from his fancy job; Diggory is an outcast because of his profession; and Thomasin is the subject of gossip and scandal after her failed elopement. Yep, society is a harsh judge in this book, given how much Egdon is ruled by the past and by very old customs and ideas. But there's a modern, urban world lurking on the fringes of the Heath and there's potential for conflict there. Egdon may be ruled by tradition and custom but the modern world is out there, and it's debatable whether or not tradition will be able to hold back change forever.
Overall, this novel reads like a fable that is removed from the present, modern world, and set in a distant, mythic past.
The real reason Thomasin married Damon was fear; she was afraid of bucking social conventions and opted to avoid a scandal and a ruined reputation.