Gender policing is the method by which gender roles are enforced and deviation punished. It's one of the most prevalent forms of social pressure and is particularly common in teenagers and young adults but happens at all stages of life. It ranges from the innocuous to the violent: parents curating the toys and interests of their children, teenage boys calling each other gay, girls labeling others as sluts and bitches. And in the extreme, corrective rape and murder.
The carrot and stick of shame for deviance and praise for conformity quickly instills these values, making people victims and then perpetrators by turns. LGBT people in particular suffer from gender policing, both from external sources and from internalized prejudice. Which is why even when surrounded by support, many find it extremely difficult to be themselves.