Saint Thomas is not a puritan. That a particular sin is not punished does not mean that it is morally licit to do it. We must distinguish between a moral perspective and the legal perspective. We must distinguish between what the state allows and what the state can and should control. We Christians must learn to follow in our behavior not the minimum of the state, but the maximum of the Gospel.
Christians live in the diaspora.
People will commit many sins which go unpunished. We don't live according to the values of the world, however. The Christians didn't go to the Colosseum to watch. We Christians likewise should avoid immoral entertainment. The decision about whether to outlaw a particular sin is a political one, not one about the moral qualification of the act.