One of the ways Stormtroopers maintain order and avoid reconditioning is by disciplining each other, mainly sexually. Stormtroopers holding down whoever broke ranks, taking turns, and humiliating them. Everyone has participated at least a couple of times—some troopers more enthusiastically than others—and the First Order looks the other way because it's good for order and morale.

When General Hux is taken prisoner by the Resistance, Finn disciplines him, Stormtrooper-style.

His actions—and the reason he thinks this was more or less "okay"—horrify everyone else.