The principle of universality is not a 'theory'. Just moral truism.

... if we adopt the principle of universality: if an action is right (or wrong) for others, it is right (or wrong) for us. Those who do not rise to the minimal moral level of applying to themselves the standards they apply to others -- more stringent ones, in fact -- plainly cannot be taken seriously when they speak of appropriateness of response; or of right and wrong, good and evil

If people cannot rise to the level of applying to ourselves the same standards we apply to others they have no right to talk about right and wrong.

If it's wrong when they do it, it's wrong when we do it.