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.
异性恋伴侣不一定要一夫一妻制才会结婚或者结婚后才一夫一妻制。一夫一妻制并不比有孩子更能定义婚姻。一夫一妻制不是强制性的，它的缺失并不意味着婚姻无效。