How do males know that their pelvic floor will rip and tear if they ejaculate sitting upright with back straight on the edge of the sofa?

They don't, because it's untrue.

thats funny

Give it up dude...just give it up

The don't know that, because that is not true.

Aren't you sick and tired of posting the same question day after day? If you are not, I know know I am of seeing it.

Males KNOW it is completely safe to ejaculate in a multitude of positions, including "sitting upright with back straight on the edge of the sofa".
You can ask this "question" as often as you'd like and reword it all you want... but it doesn't change the fact that what you are talking about is complete and utter nonsense.
If you dispute my allegation of "complete and utter nonsense", I invite you to cite whatever reliable, testable and repeatable evidence you may have.
Not thought experiments, hearsay or anecdotal accounts... but real - reliable, testable and repeatable evidence.