Found the info!!
Read this part of the borderline/ the boundary of delusion author's note
Physiologically speaking, the sigmoid colon is normally a one-way passage. When it's stimulated in the opposite direction, the brain interprets it as a danger signal, and the pituitary gland releases endorphins to counter that fear.
Omg thanks so I was sorta right on the brain releasing some stuff cause a penis is NOT SUPPOSED TO BE IN THERE and the brain freaks out low-key. Yeah it releases endorphins and adrenaline or something because it sorta but not at the same time triggers a flight or fight? So it gives like a high. And so combined with stimulation it feels good ig.

Damn they fuck a lot. Anyway if anyone is wondering what the "second" hole is it's the sigmoid colon! It's the entrance. It's not easy to reach at all there's considerable length and girth to have although some positions make it easy-er to reach. I don't remember 100% but it supposed to feel pleasurable (?) and actually activate your brain into sending some hormones and adrenaline or something after it's penetrated he's literally fucking his guts but yeah. I don't remember cause I read about it a little while ago but it was something along those lines.