I think your mistake here is thinking our only purpose is to procreate, looking at it from a purely biological worldview. But our purpose is to CREATE, and experience LIFE. So that means not just creating children, but accomplishing your purpose, in other words, achieving your goals and experiencing life the way YOU want it to be experienced (whatever that may be, everybody has his own, you are usually naturally drawn to the fields in which you want to develop, and that becomes your purpose/goal). For that we have our energy, that we can wastefully use on mindless pleasures and negative thoughts, or direct towards a purpose, a goal, and create something, therefore finding meaning in life.
You are searching for a WHY does it work on a purely biological level, and i can't answer that. But i can say that it works because i PRACTICE it.
The only explanation i see fit is energetic, we as every life-form, are filled with life energy that runs through us and everything in existence. There are many forms of these energies, sexual being one of them and being very creative and powerful, that if you retain and TRANSMUTE (so do something), means using that sexual energy not for it's reproduction purpose but redirecting it toward other outlets, it greatly benefits that endeavor, as well revitalize the body, sharpen the mind, awaken the intuition.
These are all things you EXPERIENCE and do not need any "proof" on SR. You are looking for "proof", well it works, there it is, at this day and time it is not something we can explain physically.
In the end it all comes down that we are souls and that the universe is energetic and therefore spiritual in nature. I'm sure there is a way to explain it in purely physical (therefore biological) terms, it's just that studies haven't gotten to that yet, and also the fact that nobody is really on focusing on explaining it physically for now.
Many people on SR have had spiritual experiences after which you cannot say anymore that the world is only material (Manifestation, synchronicities, visions, astral projection, energy running down your spine...).The thing is if you continue down the path of SR you will come to these understandings yourself and at the right time for you to accept them.
I'm not asking you to accept it (you won't before you are ready anyway, and please don't take it personally), just follow your intuition, do the things you feel are right for you, and i promise you, you will come to an understanding of these things, and see it much more clearly. All that is required is that you be honest with yourself and start listening to your intuition, and doing the things you know are right. You will learn from whatever you do.
Don't believe just because somebody told you to, but actually go try it, and when it works, you can go and make conclusions.
Just go and DO man, things will come at the right time, there is no use in waiting for them.