Si non caste, tamen caute. ("If you can't be good, be careful"). That is the wisdom of the world which values natural well-being over the supernatural. Hence "safe" sex means try to keep your dirty actions as clean as possible. Saint Paul (Ephesians 5:1-20, from which this Sunday's second reading is taken) says beware of this degrading wisdom of the world which presents itself as light to justify every form of lust, and is really darkness in disguise.
This worldly wisdom is even forcing it's way into our common greetings. For some years now the New York trains say "Have a 'safe' day," instead of the much more common, still, "have a good day." I have occasioned to hear parents tell their children: "be safe." "Be good!" is a much much safer direction. In other words: you will be safest when you are good. And if you cannot be safe being good, well, live dangerously, then we are in the age of martyrs! Christ was good even when it became dangerous!
Your chastity shall protect you, especially your soul. Protect your soul and your body will lack no good thing! There is no safety in the works of lust. That's death.