[KurtSF]

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We call in to question soooo many motivational tools such as money, religion, obligation to others, spite, etc, but it seems like sex is the one invincible, pure, irrefutable motivator in our species

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Seriously.

I know you don't want to get into the "its natural" argument because there isn't a good argument there. But that's about it. Your purpose as a member of the species, ultimately, is to keep the species going. You do this by having sex. Therefore, sex drive is built in, or evolved in, or however you want to look at it.

You might have more fertile ground (so to speak) investigating the quality/quantity bifurcation between male and female reproductive behavior systems.