But aren't non conforming people explicitly going against stimulus in their environments?

It depends on the individual and the environment. Stimuli don't impress a vacuum -- they impress individuals. Individuals have distinct personalities. Environments have macro and meso/micro variants. What a culture demands may be countered to one extent or another by the family or immediate environment.

A girl raised with strong received gender norms may or may not desire to climb trees, like the boys around her are doing -- depending on her individual personality. If she has no desire to climb trees, she easily adopts and internally reinforces the gendered norm re tree-climbing. If she desires to climb but her personality (for whatever reason) cannot prevail against the norms, she internally reinforces the gendered norm, perhaps more reluctantly. If she desires to climb and her personality is capable of transgressing the norm, each time she climbs a tree she remaps her internal experience in opposition to the norm and reinforces her reality as a climber of trees (this is a neuro-motor process as much as a cognitive process) -- she is choosing and reinforcing one set of stimuli (transgressive and experiential) over another (gendered socialization).

ETA

All kinds of sexual behaviour existed before porn and the internet.

It has never in human history been so widely disseminated and easily available as porn.