What consenting adults do in the privacy of the home is no business of mine, but by trying to legitimize an action publicly and/or legally makes it our business. Are these people not also trying to impose their version of morality on society?...
No they aren't. People like you are trying to impose your version of morality on them and I have a problem with that. If these people try to impose their morality on you and force you into a plural marriage arrangement against your will, then I'd have a problem with that too. But as far as I know they aren't imposing anything. They just want to live their lives in a way they believe leads to happiness.
At one time intolerant people like yoursel were trying to break up mixed race and same sex relationships. As our society becomes more tolerant of our differences, its inevitable that plural marriages will become legal.