"Youthful beauties are soft and supple,
But hidden at their waists are swords that chop off the heads of foolish men.
Although their heads may not fall from their shoulders,
The marrow dries to dust in their bones."

－Immortal Lu Dong Bin－

In this poem, Immortal Lu Dong Bin is warning us (with a bit of humor) of the dangers of over-indulgence in our sexual lusts.  The teachings of the Dao tell us that seeking balance in our lives as well as preserving and nourishing our vital energy are essential to our cultivation and evolution as balanced, healthy individuals.  One way in which people often deplete their bodies of energy is through over-indulgence in sex.  Over-indulgence leads to, among many other problems, an unpeaceful mind and an unhealthy, unbalanced, and depleted body. 

Better to focus on nourishing our vital energy and put it to practice and cultivation of our essential nature to make us stronger and healthier than to over-indulge in sexual activity and thus waste it and degrade ourselves.