In The Scarlet Letter Why did Hester have to wear The Scarlet Letter?

In the Scarlet Letter, Hester was forced to wear the red letter A indicating "adultery" (which in the strictly religious community was a sin and therefore nominally a crime). She secretly slept with the preacher in the town, and became pregnant, and because no unmarried man came forward, they correctly assumed a married one was involved. The town believed her punishment should be to wear the letter A as a sign of her sin, which made her an outcast in a society that prided itself on conformity.
Adultery.