The girls who could not cum from penetration had the permission to cum, and they concentrated very hard to try and cum from the relentless thrusting of the machine set up just so behind them.
The girls who could cum from penetration did not have the permission to cum, and they concentrated just as hard to try and NOT cum from the relentless thrusting of the machine set up just so behind them.
If you didn’t know which ones were which, you honestly could not tell the difference, until the second group started to fail, one by one.