[…] Then, if he says he loves
It fits your wisdom so far to believe it
As he in his particular act and place
May give his saying deed, which is no further
Than the main voice of Denmark goes withal.
Then weigh what loss your honor may sustain
If with too credent ear you list his songs
Or lose your heart, or your chaste treasure open
To his unmaster'd importunity.
Fear it, Ophelia; fear it, my dear sister,
And keep you in the rear of your affection,
Out of the shot and danger of desire.
Laertes tells her to guard her "chaste treasure" —not because he's interested in chastity as a moral issue (this isn't about Promise Keepers), but because he believes Ophelia's virginity is literally valuable. It'll determine what kind of marriage offers she'll get, and what kind of family she—and he—can align themselves with.
History Snack: In Shakespeare's day, there were plenty of handbooks on this matter, including Juan Vives's Education of a Christian Woman, which says a maid "hath within her a treasure without comparison." (Vives's handbook was translated from Latin and published in English in 1592.) Another handbook called A Godly Form of Household Government (1603) says that a woman's virginity is "the best portion, the greatest inheritance, and the most precious jewel" of her dowry. Why all this talk of treasure? Well, in the 16th and 17th centuries, eldest sons inherited all their fathers' wealth, titles, and lands (this is called "Primogeniture"). Marrying a virgin insured (theoretically) that a man's children were legitimate and that the family wealth could be passed on from generation to generation. So, literally, marrying a virgin was like insuring your fortune: just good business.