Straight couples don't have to be monogamous to be married or married to be monogamous. Monogamy no more defines marriage than the presence of children does. Monogamy isn't compulsory and its absence doesn't invalidate a marriage. ↗
When I need a word and do not find it in French, I select it from other tongues, and the reader has either to understand or translate me. Such is my fate. ↗