There are several synonyms for the phrase "transgress bounds of decency." One of which is "crossing the line," which conveys the idea of breaking a limit or boundary. Another synonym is "violate ethical standards," which refers to the breach of societal or moral principles. "Overstep the mark" is another alternative, which suggests stepping beyond what is socially or behaviorally acceptable. Lastly, "trespass against decorum" underscores the infringement of the norms of behavior or conduct. All of these synonyms are interchangeable with "transgress bounds of decency" and help to describe actions that fall outside the bounds of what is acceptable in a given society or culture.