People who have no desire to have offspring.

People who don't want to get fat.

How come they still want cupcakes?

Eating cupcakes don't automatically make somebody fat.

Having sex don't automatically make someone pregnant.

Sex is fun, kids aren't. I don't really get what the mystery here is.

Yes, but the whole point of heterosexual sex is to procreate. Why people who encounter in homosexual sex are able to feel sexual pleasure is another question. If a straight woman does not want to have children, what is the reason for her being horny?

Yes, but the whole point of heterosexual sex is to procreate
Not quite. Our Bonobo cousins, for instance, have sex for many different reasons not related to making babies. Sex is, besides procreation, also meant for feeling good in mind and body, to relief tension and to strengthen bonds.



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Come, lovely child! Oh come thou with me!
For many a game I will play with thee!

Humans have the desire to have sex so they will have kids and they usually have instincts to take care of them after they're here. I think the intellect deciding whether one wants kids beforehand works separately in many cases.

If a straight woman does not want to have children, what is the reason for her being horny?
Simply put, there is no other reason than the natural final end (Aristotelian) of the human biological process. From a biological perspective, pleasure exists in the sex act only to drive people to biological reproduction as its final cause. This doesn't mean that humans cannot engage in sex for pleasure, but if pro-creation was not the final end - along with what comes with it, e.g., increasing pair bonding and mutual affection which enables successful raising of children - then sexual pleasure would never have existed at all.

Love Bites. https://www.youtube.com/watch?v=tP6XE9nEVFs