You can be sued for breaking a promise to marry someone.

The Law

South Dakota is one of the few places where 'alienation of affection' and related heart-balm torts have lingering legal footprints. Historically, if a man promised to marry a woman and then backed out, she could sue him for the financial and social damages caused by the broken engagement. While rarely enforced today, it makes getting down on one knee a legally binding verbal contract.