Why weren't knights able to marry?

Knights could, and did, marry. As a knight, they would own lands: this meant a marriage could be a valuable political tool, allowing both the knight and his bride (and her family) to profit from the union. However, their prospective marriages were often subject to approval or recommendation by their overlord, so they weren't entirely free to choose whom they married.

In tales of Arthur and his Knights of the Round Table, Sir Galahad is known for his chastity and purity. This is a good example of the chivalric ideal of a knight choosing his quests and allegiances over temptations of the flesh. However, in practice, many (if not most) knights did marry.