In King Arthur's Knights, why does King Mark decide to take a wife?

The book displays a very realistic and jaded view of the world of chivalry. It is rife with adultery, characterized most visibly in Sir Tristan and the Belle Isolde. However, it should be noted that Sir Tristan had met and fallen in love with Isolde earlier, and that his uncle, King Mark, jealous of Tristan and seeking to undermine him, appears to seek marriage to Isolde for just such a hateful purpose, going so far as to ask Tristan to go and seek her hand on his behalf which Tristan, understanding that to be his knightly duty, does