In general, marriages between the well-born in the Middle Ages were arranged–daughters in particular were essentially sold off in order to cement alliances, concentrate wealth, or gain allies.  That is not to say that sons were any better off, since they too were not marrying for love.  They, however, for the most part had more leeway on whether or not they were faithful to their wives, and certainly had more freedom in general.  Perhaps the most startling example of an arranged marriage that cements an alliance is when Isabella de Braose married Dafydd ap Llywelyn, the son of Llywelyn ap Gruffydd, the Prince of Wales, after her father, William, had been hung by Llywelyn for sleeping with Joanna, his wife.

The families trees of the Gwynedd indicate how closely linked the Welsh princes were to the Marcher lords and to the Kings of England themselves.  Click on the image to make it larger.