I could probably summarize opinions of those voted against same-sex marriage in "marriage and any tax benefits coming from it are there to support people who will eventually raise children". I see, that gay people are not happy, because opposite-sex marriages still get some benefits, but some of them do not end up having kids.
In my opinion, to separate the bugs from the meatloafs, we should separate living together and having kids legally. And the first one should be available to any kinds of partners, but should not provide any benefits, and the second should give benefits, and be positively recognized in society as "family".
But allowing gay-marriages is in no way makes lives closer to that.
Ed: in this case brendan eich either made a mistake with his donation, or is anti gay rights.
The tax breaks for raising children (e.g., deductions for dependents, child tax credit, etc.) aren't tied to marriage, they are tied to having legally-dependent children.
Insofar as there are tax breaks for marriage rather than having children, there's a long discussion possible about what the public policy rationale for them is, but the best explanation is probably incentive to the social stability of a stabilizing relationship and the reduced risk of dependency on public support programs that comes from a mutually supportive relationship (which, even if it fails, still often comes with legal support obligations.)
These are just as valid with childless, including same-sex, marriage as with any other marriage.