So back to the topic, no one has any idea how LGBTs voted pre-1992? I mean it's not like the Democratic Party has been the clearly more pro-LGBT party before about 2000 (which is the OP's point).
Still I think that the gay community tended to drift towards the Democrats by default beginning probably in the 70s just because they tended to be clustered in urban areas and would have been more likely to form pressure groups within the Democratic Party, Reagan's reaction to AIDS I think was the tipping point even if Democrats in the 80s were really just as bad.
I could see Ford possibly doing well in 76 just because Carter was the one perceived as a bible thumper and probably seen as less gay friendly at that point.
I think the rise of the Religious Right in the late 70s/early 80s and its clear association with the Republican Party was a significant factor in pushing LGBT into the Democratic camp. Reagan was the first Republican President to be associated with Religious Right (probably at least a partial explanation for his reaction to the AIDS crisis). Gay people no doubt observed this and responded accordingly.
LGBT activists noticed how the Civil Rights and Women's Rights Movements had successfully operationalized their policy goals through the Democratic Party and figured this could work for them as well. The Party was not very supportive at first -- but certainly blacks and feminists faced resistance within the Party when they began pushing for change as well.
