In Le Morte D'Arthur, knights demonstrate their loyalty to their overlord above all by being willing to go to battle for him, and by refusing to engage with him or his other knights. So if Arthur tells you to drop everything and go fight some guy you've never even met, well you'd better hop to it, mister. In fact this definition of loyalty is so extreme, and so powerful, that Launcelot refuses to fight with Arthur even when the king invades his lands, choosing to wait until he can claim self-defense in order to keep his honor intact. Le Morte reserves its highest praise for characters like Launcelot and Trystram, who don't allow their roles as lovers to get in the way of their loyalty to their respective kings. And it's knights like these, who can balance the roles of lover and knight with relative success, that have the greatest reputations in the Round Table.
You might say that Trystram and Launcelot successfully demonstrate loyalty to their kings. But you'd be wrong. In fact, Trystram's and Launcelot's love for their queens makes it impossible for them to be truly loyal knights to their kings.
Launcelot's failure in the Grail Quest demonstrates that loyalty to a woman prevents true loyalty to God, which tells us that a good Christian knight had better be chaste (like Galahad).