The phrase "sinful but legal" implies that an action is permissible by law, but morally questionable. There are several synonyms that could be used to describe such actions, including debauched, licentious, immoral, indecent, and vulgar. These words suggest that the action in question is not only questionable in terms of morality, but also lacks social acceptability. Other terms that could be used include lewd, lascivious, dissolute, and corrupt. These synonyms imply that the action in question violates societal norms, and may lead to negative consequences for the individual or group involved. Despite being legal, actions that are labeled as "sinful but legal" can still be harmful to individuals or society as a whole.