465. (3) It is polygamy, which has been banned from the Christian world, and ought to be banned. Even though it is not recognized as such, because it has not been so stated and thus defined by some law, everyone sees, even without keen perception, that taking a mistress simultaneously or conjointly with the wife is polygamy. For the woman is a kind of extra wife and a sharer of the marriage bed. As for the point that polygamy has been banned and ought to be banned from the Christian world, this we established in the chapter on polygamy, especially by the following considerations there: that it is not lawful for a Christian to have more than one wife (no. 338); that if a Christian takes more than one, he commits not only natural adultery but spiritual adultery as well (no. 339); and that polygamy was permitted to the Israelite nation because in it the Christian Church did not exist (no. 340). It is apparent from this that to take a mistress in addition to one's wife and to share a bed with each is a foul form of polygamy.