This IS the holding of the case. He boiled down what he considered the defendants' argument to be ruled on it.
And the absence of a rational basis is a finding of fact. This decision doesn't rule on that for cases of polygamy. Under this ruling, you have to find a basis other than moral or traditional concerns for prohibitions of polygamy to stand. This case doesn't dispose of that issue; it invites it. It invites litigating the whole issue as to whether there IS a rational basis for such a prohibition, because any moral or traditional reasons are now out the window.
So yeah, it's now a very live question, much moreso than it was before this decision, 'coz now you have to get past the "weird" factor for banning polygamy. You have to get into the facts.
Before this, a state defining marriage under traditional, opposite-sex, two-person terms took care of the question. No more.