Thank you for your very good question!
Since we have knowledge, gyan; I can speak about this topic with greater confidence than explaining my answer to someone who is trapped in the duality of words and “black or white morality/understanding.”
Let me explain myself:
For most you have either “good,” or “bad.” “Hell or heaven,” “black or white,” “sin or virtue.”
Here is my first point to explain your question:
There is nothing sinful in the Drama. In the Drama everything that happens is just necessary. For it repeats, eternally. The value of those acts, change in time.
Note that I am not making up stuff here, but just logically going with lines of “pure” gyan.
We experience the full range of experiences.
However, “timing” is the difference. That timing differs from every soul.
Since we are out of touch with our original state of spiritual bliss, we had to come up with “morality,” to maintain some control and order in our society. Therefore government with rules and regulations and religions with dogma and commandments. Then, the words “sin” and “virtue” came up to divide things.
Because we do not recognize this state of spiritual bliss, we have to be treated like “little children,” and told what is “good and what is bad.” A typical sakar Murli will go into those aspects, until we spiritually “grow up.”
Therefore, is sex a sin? NO. It is a virtue? NO. It is just what it is. Sex.
In gyan, we recognize that everything that we perform has an effect- Law of karma. This Drama is “cause and effect.” Therefore, sex has an effect. That effect we can call “good or bad” according to those consequences: happiness or suffering.
Is unwanted pregnancy good or bad? Are sexual transmitted diseases, good or bad? Is sexual addiction good or bad? Is pornography good or bad? Is child bearing without parental values, good or bad? and finally (even though my list could go on) Is “sex-lust,” good or bad?
That is for you to “decide.” I am just giving you the logical facts “at this time.”
Therefore, some people choose to remain celibate and avoid “playing with consequences,” others, on the other hand; want to experience sex lust and are OK with the consequences. That is how the world gets populated as you rightly mentioned. That is their role.
However, for a Brahmin soul, the objective of this life is to become soul conscious and God conscious.That is it.
Definitively “playing with sex lust,” will not get you there.
A human being with vices (lust,) cannot make sex a vice-less “act;” thus sex-lust.
It is all cause and effect. You “choose.” You live with those consequences.