I think there's some great discussion here, thanks everyone for sharing.
i do not think there is one simple, standard answer to this question. i subscribe to the theory that human sexuality is a spectrum. i've come to see that spectrum as multidimensional vs just linear. We all land somewhere on that spectrum and find others who live in the same neighborhood, but there are no two alike, we're all individuals. i do not believe there is 'a reason' for who and how we are sexually, but many reasons/causes working in concert to play our particular tune. i think one of the reasons humans need and seek relationship is it aids self exploration and self discovery. We can self reflect, but then we only have one (biased) perspective. We also can see ourselves reflected in and by others. Also, we are not static beings. Life is fluid. i think self discovery, as well as learning others in relationship, is a never ending, life long process and the only 'real' stasis is death.
Sorry for the long prelude lol. Here's some physiology/science information. i think it's telling that the same nerve that conveys pleasure and is connected to the penis and vagina, is also connected to the anus and rectum. We perceive/experience pain and pleasure in the brain, and i think that's where things get more complex and individual.
"The pudendal nerve is a major nerve in your pelvic region. This nerve sends movement (motor) and sensation information from your genital area. The pudendal nerve runs through .your pelvic floor muscles that support organs and ends at your external genitalia.
The pudendal nerve is crucial for sensation and function in your pelvic region, including the genitals and anus. This nerve is part of your peripheral nervous system. Peripheral nerves send signals from your central nervous system (brain and spine) to your limbs and certain organs.
The pudendal nerve’s motor function controls the movement of your:
Anal sphincter muscles, which help you hold in or release feces (poop).
Urethral sphincter muscles, which help you hold in or release urine (pee).
The pudendal nerve also provides sensory information about touch, pleasure, pain and temperature to your:
Penis (part of the male reproductive system).
Vagina (part of the female reproductive system).
Perineum (the skin between your anus and your vagina or penis).
Anal canal and anus (part of your digestive system)."
https://my.clevelandclinic.org/health/body/22000-pudendal-nerve