No offense, I don't think this is a legitimate concern because an erect penis will make room for blood flow.
The amount of pressure is also relative (should be reasonable) and the pussy shares equal amount of pressure.
Saying that the penis falling off is going too far. It's up to the person to inflate it just enough to maintain a tie.
In reality, ties are not 100% secure and dogs do separate when mating is finished.
So if it's a concern, inflate just lightly.
I just don't want this thinking preventing potential inventors from getting past this issue.