Arranged Marriage- Iron clad contract that was drafted to unite two families/countries before the children were born.
Gambling father loses contract in a game of cards.
Conquest - Leader/General/King/Queen of an invading force takes the prince/princess as their prize to secure and legitimize their claim on the kingdom.
Just Pretending - young professional (male or female) takes home a much older and sophisticated friend of a friend who is pretending to be their significant other for the holiday.