Every person, beginning in puberty and continuing for some time thereafter, makes up their mind about what they are living for. Most people make this decision quite consciously early in life and then stay the course.
However, they are often unaware of why they chose a certain religion, ideology, or philosophy.
The point is that people largely make decisions about their "purpose" in life based on their desire to be accepted members of their communities. For young people, it is of the utmost importance to be regarded as "high quality" among their peers, as their mating prospects depend on it. During puberty and in the years that follow, mating prospects are the one thing that matters most in life.
It is often just a trade-off: forgo mating opportunities in the short term, and thereby earn respect within your community. If you are well-respected (for your abstention), then your market value will rise, and your mating prospects later on will be better.
No matter how you look at it, the only thing that really matters, for young people as well as later in life, is mating opportunities. If awareness of this connection between esoteric decisions and biological drives becomes more widespread, more people—especially young people—will hopefully choose the direct approach (sexual liberation) rather than the indirect one (conforming to religious, ideological, or philosophical trends in order to improve their market value and later be rewarded with mating opportunities).