What is the moral of this story?

As you see in both the title and the fact that virtue is a major theme....... I believe that holding onto one's virtue and the ability to defend it at all times without question is the story's moral. If you need to look further into the story's themes, simply follow the link below;

The Nature of Virtue

Richardson’s novel has often given the impression of defining “virtue” too narrowly and negatively, as the physical condition of virginity before marriage. The novel’s conception of virtue is actually more capacious than its detractors have allowed, however. To begin with, Pamela makes a sensible distinction between losing her virginity involuntarily and acquiescing in a seduction. Only the latter would be a transgression against sexual virtue. Moreover, almost the entire second half of the novel is taken up with the explication and praise of Pamela’s positive qualities of generosity and benevolence. Mr. B. values these qualities, and they have brought him to propose marriage: reading her journal, he has discovered her genuine goodwill toward him, particularly in her rejoicing over his escape from death by drowning. As a result, Pamela's active goodness merits the “reward” of a happy marriage as much as her defense of her virginity.