A knife, spear, wooden shaft etc is different than a penis. The penis, no matter how hard, also has give to it. Yes, the woman's body gives way-no shit, but not that much. Otherwise it would be fucking painful and you would see the girl scrambling for 911, not moaning in lovely acted porn bliss.
First of all, I'm not stating that it happens in each and every case, but certainly there are instances where it can and does happen, and this is a thread dedicated to that type of occurrence, ergo the majority of the videos posted here are likely to have it. But if you take the videos in this thread (and the other threads that have been closed) at 100% valid and compare the number to ALL of the pornographic movies in existence then you'd see that the number of genuine occurrences is actually quite small. Clearly all circumstance have to be just right in order for the "belly bump" to appear. Size of penetrating object, be it a penis, dildo, hand, etc.; overall physical size of the female, it would be much more evident on an very thin woman as opposing to a woman of more significant size and angle of penetration. Get all the right ingredients together at the right time and you'll make magic every time.
...If ANYTHING, it is the penis pushing the uterus forward...
If it's the penis pushing the uterus forward, then where does the uterus go one wonders? Into some magical cavern that exists not in time nor space but in some other dimension, only to be found by those that already know where it is? Certainly the uterus isn't cast in stone as to be an immovable object for, as you stated "it would be fucking painful and you would see the girl [and likely the male] scrambling for 911, not moaning in lovely acted porn bliss."
And watch the huge dildo videos. Those girls take dildos that make Mandingo look like a gnats pecker and you don't see their belly "poking out".
So what you're saying here is that if it doesn't happen in every time it doesn't ever happen at all? That's not a very convincing argument, but apparently you're quite happy with it.
And in closing, while I've merely presented a reasonable, objective opinion, positing the "belly bump" you've decided to lower yourself to present a very biased and infantile retort. And since there are people reading this thread for the enjoyment of a specific fetish this will be my last word on the subject.
PS Perhaps you need to combine "geeky physics examinations" into your analysis of biology instead of acting like a person with any in-depth knowledge of the human anatomy, or am I mistaken and you're actually Dr. Ramsey, M.D.?
Have a happy day!