The contrast between legislating and legalizing social change is probably one of the most polarizing debates in the United States.
First abortion, then gay rights -- Roe v. Wade actually happened, while your parallel WI didn't (but might have followed similar lines to Roe.)
The judicial means of legalizing abortion in the US bypassed the legislatures and the views of many who consider abortion immoral and/or even murder. While Roe pleased those who had argued that a woman's personal soverignity was clearly contained in the Constitution, many though that such a decision should be of the people. As pro-life advocates might argue, if the states were to decide on their own the "silent majority" would defeat pro-abortion measures. Hence, one of the, if not the, main objectives of the pro-life movement is to reverse Roe and then end abortion through selective state legislation.
I think that a national legalization of gay marriage in 1972 through judicial means would likewise have deeply polarized the nation as much if not even more than abortion. There would be a similar split between the concepts of "autonomous rights" versus morality. The question of judicial or legislative primacy over social change would become even more pronounced. The presence of two highly controversial judicial rulings might spark an earlier evangelical and Catholic political union with the GOP. Combine this with Nixon's Southern Strategy, and the potential for a very strong and earlier social conservative movement in the GOP becomes quite possible. The social conservative versus secular political stratification seen in modern US politics might have begun even earlier than Reagan; and the litmus tests and gridlock in Congress over "values issues" would have characterized much of the 70's.
From a social front, expect massive civil disobedience to the "gay marriage" law in many states (although NY, maybe parts of New England, and parts of the West Coast might perform gay marriages.) There is a precident for this in state abortion law: some states effectively "ban abortion" by having only one abortion clinic in the state. Should gay marriage be legalized in 1972, expect there to be very few places gay couples could go to be wed.