Why do your internal muscles and nerves between anus and scrotum rip at orgasm if seated upright with back straight?

They don't, they won't and they never will.
In fact, I would need to see evidence of such a thing before I'd even consider it as something that could happen.

they dont

They don't.

Forget the nonsense you have read or thought up about tearing pubic nerves as it is not true.

Whilst one usually seems to get most pleasure from masturbation when prone, it can safely be done standing, sitting or however yo wish.

Um-- They don't do that. Many guys have sex sitting in a chair or on the toilet.

Because that's what happened to your brain. Now you think that's what happens to other parts of the body.

You're back. Where've you been for the past few weeks?