Voiding waste has obvious biological importance. What would happen if an organism didnt remove their waste products?
Well in the case of humans, we would literally be filled with shit.
Then what? We wouldnt be able to ingest anything new because our guts would be filled to capacity.
Now the gut is constantly moving food towards the exit.
As the end of the line gets filled up, it stretches and we can feel this and the associated pressure internally. So voiding wastes releases the pressure.
And yes in the case of an really big turd you can stimulate your prostate.
The muscles around the prostate contract during ejaculation, and when you manually stimulate the prostate with fingers or even a turd, you are mimicking the process of ejaculation. The prostate plays an important role in the production of seminal fluid and is involved in the "release" of ejaculate. And as you know, ejaculation feels good.
No need to question your sexuality.