I think we must distinguish sharply between marriage as a legal institution and as a cultural/religious custom. The former is a matter of equality and civil rights, but it can be satisfied by the institution of civil partnership. As for the latter - cultural customs and rituals are not rights and don't have to be inclusive.
It's not discrimination to have a custom specific to celebrating the relationship between a man and a woman, which is what marriage has been in our culture until now. Indeed, even in past cultures in which homosexuality was considered completely normal and wasn't discriminated against at all, such as ancient Rome, marriage was considered a heterosexual custom and gays didn't want to take part in it any more than Christians feel excluded from Bar Mitzvah, for lack of a better comparison.
And if that seems too distant an example, then how about the 1980s? Some of the earliest gay marriage advocates (was it Andrew Sullivan? I'm not sure) were met with criticism in the gay community itself, because some of its members didn't like the idea and considered it contrary to their identity. It is all a matter mindset and I feel like it's a legitimate cultural dilemma - not an issue of equality.
I can empathize with people who are attached to the traditional definition of marriage and its symbolism. I don't think you have to carry any ill-feelings towards gays in order to want preserve it and I think it's the case for a significant proportion of gay marriage opponents.
This point of view seems quite obvious to me and it's unfortunate that almost no gay marriage supporters recognize it, as it causes unnecessary polarization of the debate. It is a disservice to both the LGBT community, who feel more threatened than they should, and to some of the defenders of traditional marriage, who are undeservedly labelled bigots, ostracized, and even - as in the instance of Brendan Eich - disenfranchised for their cultural/religious beliefs.