What was unusual about Edgar Allan Poe's marriage?

Poe was 27 when he married his first cousin, Virginia Clemm, his only wife, who was 13. Marrying young was common then; Poe's natural mother Elizabeth Arnold married her first husband Charles Hopkins when she was 15 though he was only 17 (I believe), and they were not cousins.
Edgar Allan Poe married Virginian Clemm, on September 22, 1835.

She was his first cousin and only 13 years old at the time of their wedding.