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There is no defined "purpose" to an individuals life.
The main purpose of every single fertile person ever is to produce healthy and fertile offspring.
The main reason for men being born into life is because people had sex. Some men are born using other methods, such as artificial insemination. I think you mean to say "purpose" or "goal", not "reason".
An infertile man's main "reason" of life is to survive.
Evolution is geared towards furthering a genetic line
the real reason behind it all is to further science
This only applies if the goal of said sex is reproduction, then it's simply a matter of following ones biological imperative.
sex is for Gene spread.sex is process, spread gene is purpose.