Polygamous is the adjective for a person, a marriage, a family, or a custom where someone has more than one husband or wife at the same time. You say a polygamous household, not “a polygamous” when you mean the practice itself — that noun is polygamy. It is a formal, book-and-news word, not playground talk. Though actually, in loose speech people often hear only “one man, several wives,” but the English adjective also covers a woman with more than one husband. It is about marriage, not about secretly seeing someone else, and not about dating several people without being married.