Historically, a betrothal was a binding contract - that's why a jilted partner could sue for "breach of promise". There's some interesting stuff on Wikipedia, including this:
"In some historical cultures (including colonial North America), the betrothal was essentially a trial marriage, with marriage only being required in cases of conception of a child. In almost all cultures there is a loosening of restrictions against physical contact between partners, even in cultures which would normally otherwise have strong prohibitions against it."I was looking up some Swiss records last week, and found a child born in 1826, shortly before her parents' marriage. She was baptised with the father's surname, and there was a note in the register to say she was "née sous la foi de promesse de marriage". (Literally "born in the faith of a promise of mariage".)
In this area, they went even further: a man who promised to marry a woman and subsequently got her pregnant could be forced by the local court to marry her if he tried to back out! We found one case where the guy had done a runner, and his brother was obliged to stand in for him at the wedding....

(The lady in question then had her baby legitimately, and afterwards was allowed to divorce the guy for desertion while still preserving her good name!)
Rambler