Sex. What an exquisite topic.
How do you define sexual immorality/indecency anyway? Bare breasts, for instance, were socially accepted in many civilizations across the world in the past. Ancient Egyptian women wore elaborate dresses that could either cover the breasts of leave them exposed. Today, you just have to walk into your nearest museum to see explicit pictures and statues of Greek goddesses. As for man, take a look at Michelangelo's David-- exhibiting your phallus so openly isn't exactly decent by today's standard. So if bare breasts and genital-flashing were accepted as a norm in the past, how can we be sure of the exact definition of sexual immorality today? What deemed inappropriate today might not be appropriate in the next hundred years.
Anyway, excuse my random rant there.
We are often taught by our parents to avoid premarital sex.
Of course, religious institutions play a huge role in making sure that young, unmarried couples do not give in to the temptation of lust because, according to them, it's bad for the youngsters. And they'll follow up their claims with allegories of broken families or couples who'd committed themselves illegally in the premarital breeding program.