David Friedman shares an interesting tidbit.
…In the early 20th century, a common pattern was for engaged couples to have sex with the understanding that if the woman got pregnant they would get married; evidence from several late 19th century European cities suggests that about a third of brides were pregnant. One problem was the risk of that the man, having gotten the sex, would dump his fiancee instead of marrying her. One solution to that, in U.S. law, was the tort action for breach of promise to marry. In a society where marriage was the main career open to women and the fact that a woman was known not to be a virgin substantially reduced her marriage prospects, seduction could impose substantial costs and result in a substantial damage payment.
Starting in 1935 in Indiana, U.S. states started altering their laws to abolish the action for breach of promise. Women responded, by Brinig's account, by requiring a down payment from their fiancees in the form of an expensive ring—which forfeited if the fiancee terminated the engagement. Think of it as a performance bond.