How early could gay marriage have been legalized without being banned by a constitutional amendment?

[Kahane's Grave Is A Gender-Neutral Bathroom]

Obviously if the supreme court ruled in like the 70s it would be, but I'm curious at which point it would have been hard for the right to get rid of it.

[Agonized-Statism]

IIRC, the US was on its way to accepting gays and lesbians at least before AIDS, and that's the tricky part with placing a Supreme Court win in the '70s. A lot of the stigma around AIDS faded with FDA approval of the first oral, non-blood test in 1994, the decline of hospitalizations in 1995 with new medications and the introduction of HAART, and the advent of home testing kits and urine tests in 1996. But then there was the Republican Revolution in congress at that same time and a whole lot of political capital gained for the religious right with 9/11, so it's iffy. Late 2000s is the surest bet.

[Skill and Chance]

It obviously couldn't have happened prior to Loving.  I think it could have happened as a natural extension of Loving at some point in the 1970's if Humphrey won in 1968.  Remember, he would get to fill 4 SCOTUS seats in his first term, and with clearly liberal-leaning senate.  At that point, I think it could have survived the constitutional amendment process narrowly thanks to the NE having many small states.  If it happens between 1980 and 2010, it probably does get amended away, however.   

[If my soul was made of stone]

I would simply hire another nation's intelligence agency to arrange an "accident" for Anita Bryant and Phyllis Schlafly sometime before Jimmy Peanuts took office.

[SInNYC]

Maybe my sample is bad having been in the south back then, but I think there is no way in hell any form of gay rights would have been adopted by the people in the late 70s or early 80s, and there would have been strong support for amendments banning it in a good chunk of the country. But 3/4 of states is always hard, so maybe it would have been state bans in most states and some form of DOMA.

I have personal awareness of policies regardng [opposite sex] dorm visitation being driven by 'what works best for keeping gays away from your floor/dorm'. People who prefered tighter policies on visitation drove the push for looser visitation policies since tighter policies generally attracted gays.