It is certainly true that moral perceptions have significantly, fundamentally changed on a number of social issues or behaviors since 2001 — most notably, gay/lesbian relations, having a baby outside of wedlock, sex between unmarried men and women, and divorce. But these attitudinal changes did not occur in isolation. They have occurred alongside important cultural and legal changes, including the rising propensity of divorce following changes to state laws at the end of the 20th century and the gay rights movement that ultimately succeeded in legalizing same-sex marriage in 2015, to name a few.
No such shifts have occurred with respect to polygamy. It remains illegal in all 50 states. And only this year, the state House of Utah, a state that outlawed the practice in 1895 in order to gain admission into the union, passed a bill that would, among other things, increase the penalties for convicted polygamists. And while academic research finds that covert polygamous marriages do exist in the U.S., they are uncommon and are largely confined to some immigrant Muslim groups and Mormon sects that have broken away from the mainstream church.