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. ↗
The real reason to oppose increasing tax rates on the wealthy is that it's a good bet they could do more to help the economy if they keep their money rather than have their earnings confiscated by the government and spent on another round of stimulus. ↗