system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are humans innately sexual throughout their entire lives?

Now I'm 25, almost 26, and this is a topic that's always kind of confused me. Later in high school I learned that boys can get erections before puberty, even before birth; and that both boys and girls can climax before puberty. As far as I'm aware, we're the only animals that are like this, but why?

Edit: To add onto this, why are we also capable of sexual activity well past our sexual prime?
gpt
As an anthropologist, I’ve always thought the answer to this stems from one of our closest evolutionary relatives, the Bonobo.  Bonobos are also sexual throughout their entire lifespan, and juvenile sexual “play” happens quite a lot.  Like us, Bonobos have sex frequently, in a variety of positions, engage in homosexual behavior, and seem to enjoy themselves quite a lot.  However, *also like us, sex seems to serve a different purpose in Bonobo society than pure reproduction.*  In Bonobos, it functions to soothe aggressions between individuals.  Someone’s upset? Have sex with them.  Want someone’s banana? Don’t just take it and beat them up, have a little sex with them.  Infant is fussing? Masturbate them a bit to calm them down.

Flip to humans.  We also engage in a diversity of sexual acts (unlike other primates), and sexuality also serves a different purpose in our society than just pure reproduction.  It’s not mitigate aggression, but it probably evolved in its current state to form bonds between two individuals or a group of individuals (not saying that’s how we always use it, but that’s probably what it evolved for).  

In short, in evolution, moving sex out of a purely reproductive context has had all sorts of downstream effects, including high levels of sexuality, even when reproduction is taken off the table.