If we exclude all the practical and legal aspects that are satisfied by a civil union, marriage is a ritual, a cultural custom. Its value and purpose lie in the symbolism ascribed to it. But customs aren't rights and they don't have to be inclusive.
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.