O, she is rich in beauty, only poor
That, when she dies, with beauty dies her store.
Then she hath sworn that she will still live chaste?
She hath, and in that sparing makes huge waste;
For beauty starved with her severity
Cuts beauty off from all posterity.
Romeo uses a metaphor of wealth and spending to suggest that Rosaline's vow of chastity is akin to hoarding ("sparing") her "rich[es]" (her "beauty). By refusing to have sex and, therefore, children who might carry on her legacy, Rosaline is basically "wast[ing]" her "beauty," which will "die" with her instead of living on in her children.
We see the same kind of metaphor at work in Shakespeare's "procreation" sonnets (Sonnets 1-17), where the poet urges his friend to have children instead of being miserly with his beauty.
Compare Romeo's speech above to Sonnet #4, below:
Unthrifty loveliness, why dost thou spend
Upon thyself thy beauty's legacy?
Nature's bequest gives nothing but doth lend,
And being frank she lends to those are free.
Then, beauteous niggard, why dost thou abuse
The bounteous largess given thee to give?
Profitless usurer, why dost thou use
So great a sum of sums, yet canst not live?
For having traffic with thyself alone,
Thou of thyself thy sweet self dost deceive.
Then how, when nature calls thee to be gone,
What acceptable audit canst thou leave?
Thy unused beauty must be tomb'd with thee,
Which, used, lives th' executor to be. (Sonnet #4)
We're pretty sure there's some version of this pickup line still in use today.