What has been the symbolism of marriage in our culture? Part of it is definitely gender-neutral: celebration of a couple's love and their commitment to each other. But it has also been about the particular dynamic and beauty of a relationship between a man and a woman, including procreation. This part is very important to many people and it is taken away if the custom of marriage is changed to include gay couples.
I think this is a legitimate cultural dilemma where both sides can be empathized with. Opposing gay marriage (but not legally equivalent civil unions) can be a legitimate position that doesn't necessarily imply any sort of intolerance or hostility towards gays. Moreover, it is intolerant not to respect this position. I find it unacceptable for people to be ostracized and disenfranchised just for wanting to preserve a custom which they hold dear and which is part of their cultural identity.
You may think it's all just academic, but I'm actually convinced that a significant proportion (though probably not the majority) of gay marriage opponents isn't anti-gay. The unnecessary polarization of the debate doesn't serve good to the gay community either, as they feel more threatened than they should - such as in the case of Brendan Eich becoming Mozilla's CEO.
As for me, I think marriage shouldn't to be sanctioned by the state any more. Civil unions for everybody and let the cultural stuff be figured out organically.