As someone who personally views the institution of marriage as bogus, I believe the second best alternative is to accept all kinds of lifestyles. Illegality of such lifestyles only breeds the worst of said lifestyles, and harmful behavior can be brought to light rather than hidden. Whether it's two men and a woman, two woman and a man, or even five women and four men, I believe love in all forms has its place in our society, and it is morally wrong for our region to restrict that.
While I agree with you on all of that, I am curious why you've proposed to expand marriage, rather than eliminate the institution entirely?