We can hardly read a word of Le Morte D'Arthur without somebody betraying somebody else. Knights betray their lords. Ladies betray their knights. It's no wonder Camelot has some serious trust issues. For the most part, our narrator heaps a whole lot of scorn on the men who betray their kings (like, say Mordred), or the kings who betray their vassals (like King Mark). But there is one type of betrayal that doesn't seem to bother our narrator all that much – adultery. Nevermind the fact that, in more ways than one, adultery is what causes Camelot to fall to pieces.
This double standard just might reflect Le Morte's borderline obsession with the vassal-overlord relationship. Husband-wife bonds pale in comparison to those of a knight and his king. In the values-system of Le Morte, in which stability depends on the loyalty of knights to their king and vice-versa, the tangled romances between men and women take a backseat to this more important connection.
Plus, Le Morte's tolerance of cheating hearts just might give us an idea of the social context in which Le Morte takes place. Back then, marriage was changing from being a strictly financial arrangement to more of a love-connection. So perhaps all this two-timing is not so much a betrayal as a symptom of people caught between two very different ideas of the proper relationship between men and women.
Le Morte D'Arthur portrays betrayal between vassals and overlords as much worse than betrayal between husbands and wives.
Le Morte D'Arthur uses Isode and Trystram's betrayals of one another to show just how loyal Launcelot and Gwenyvere are to each other.