Deuteronomy 22:28

"If a man find a damsel that is a virgin, that is not betrothed, and lay hold on her, and lie with her, and they be found; then the man that lay with her shall give unto the damsel's father fifty shekels of silver, and she shall be his wife, because he hath humbled her; he may not put her away all his days."

For further comment on this see under Exodus 22:16. Notice that in all of these regulations betrothal is EQUIVALENT to marriage. Under the Jewish customs, betrothal was, in fact, marriage, despite the fact of the bride and the bridegroom not living together for some considerable time afterward. The example of this in the case of Mary and Joseph is repeatedly evident in the N.T.