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We have a defined sexuality, which can be male or female, or other anomalies, even before we are born. Now, our sexual response is other thing, we can see kids kissing, that is more a secondary thing if the real purpose was to fulfill something, his desires to form part of something bigger and more complex with someone, although, their minds aren't prepared for that at an ethical nor operational level (a level that they are aware of the situation and can abstract it). The human sexual response is turned on since the human can feel sexual pleasure (basically since we have our nervous system and reproductive system), and we feel sexual pleasure because it makes the whole intercourse process a pleasant experience, wanting us to procreate more and more. Now, why the females become sexual before they become fertile? Maybe it's a bit hard to create a whole new system inside our body once the time has passed and we need it, and because the older we get, more time is needed to create tissue (and because all of the dangers our ancestors had, it was better to have it the sooner, so they don't need to wait the system to create and become functional to procreate and spread the species)
Why we can feel pleasure since kids and not when we become fertile? Because our nervous system is needed for that, and our nervous system take time to regenerate/create and become functional, at least in the level of complexity of the reproductive system. Other thing is the urge to fuck everything we see at a certain age, but that is for other day, i'm tired and have to study a fuckton of things tomorrow.
Good night good people.