Sex has a biological function which is used to accomplish a purpose which is reproduction.
Not in this case as the purpose, reproduction, is containing within a specific function of a body with no natural alternative in many species.
Poor comparison as as sight is merely one sensory function out of many which sex is not.
This is product of a large and more complex brain than sex itself.
There are no alternative to reproduction. You are conflating utility with purpose.
Sorry, you are very badly informed about this, while trying to sound as if you really know what you are talking about.
Yes, sex is how we reproduce, and without sex we would not reproduce. But that doesn't mean by a long stretch that that is only what sex is for. You can study humans, and our close relatives the bonobos (pygmy chimpanzees) and see that sex is also very much a romantic glue that keeps parents together in mutual support during the infancy of offspring. (That part much more in humans than bonobos, because bonobos don't pair bond, but bring up infants as a community, primarily by their mothers). But sex also has a very social purpose, and that more in bonobos than humans. It is used as a way to "be nice to one another."
Bonobos use sexual activity in a variety of ways, such as greeting, forming social bonds, conflict resolution, and post-conflict reconciliation. They are the only non-human animal to have been observed engaging in tongue kissing, in typically engaging in face-to-face genital sex.
When a new source of food is discovered, there will often be communal sexual activity, which seems to decrease tension and encourage peaceful feeding.
Bonobo females have much larger and more external clitorises than most mammals, and many times each day, in many different pairings, females will press their genitals together in what is called genito-genital (GG) rubbing. This sexual activity, which is certainly not about reproduction, happens within the immediate female bonobo community and sometimes outside of it. Females will do this, on average, about once every two hours.
Bonobo males engage in various forms of male–male genital behavior, too, most often one mounting the other in a similar fashion to hetero mounting: one will sit passively on his back while the other male thrusts on him. " They often hang together from a tree and engage in “penis fencing,” and have been observed masturbating one another and engaging in oral sex. Males also tongue kiss together sometimes. Male-to-male sexual activity is less frequent than female-to-female.
So clearly, sex, which may be reproductive, also has other functions.
