I have a theory about why t rex has such short arms (not addressed in this piece).
As we can see in todays animals, a female can hurt a male if his advances are not accepted. Female t rexes were a lot larger than males. A spurned male could become lunch.
Also in todays animals, you can sooth and put a croc to sleep by stroking his throat and belly.
There are several reasons animals have features. One is display - like a peacock has those display feathers to show other peacocks how big and beautiful he is. Or frogs can expand their throat to ribbit and attract mates.
One is defensive (or offensive, or both). Like an elephant will flap his ears, as a warning.
Or for cooling and heating - again, an elephant flaps his ears.
So why did t rex keep his short arms? He couldn't hold food and eat it, they wont reach his mouth. No good for fighting and worthless for balance.
So my proposed theory is males stroked the females throat and or belly to calm and sooth her, so he could have his wicked way with her and keep his life.