That doesn't make any sense. The brothers were living together when one of them died, leaving a widow. C'mon, man. Stop resisting this. The simple truth is that God allowed polygamy, and even required it under some circumstances. If that's true, and it is, then it follows that polygamy can't be inherently sinful. As my Catholic background theology might put it, it might break the ecclesiastical law (no small matter for Catholics, at least), but it doesn't violate the Natural Law (Aquinas states this clearly).
Sodomy is a gross violation of the Natural Law. Unlike polygamy, it is inherently sinful.
As a people, we Americans have turned our backs on the Natural Law and made a huge step toward Hell. Morally speaking, we passed polygamy long ago on the road to Hell. It's just taking the legal theoreticians a little time to figure that out.
I could tell you to “stop resisting this” too. There is no approval of polygamy in the Bible at all, even in the special case of levirate marriage.
Nor is there any mention of so-called “Natural Law” in the Bible; that’s derived from the philosophies of people like Aristotle and Plato.