I find it laughable that a candidate’s support for gay marriage should be the reason for not voting for him. Whether you want more social services for the community or a government that is better run that provides fewer services, to use a candidate’s support for gay marriage as the deciding factor is ridiculous.
If you are concerned about social norms that may affect the community, why use only gay marriage? How about voting based on whether the candidate supports the right for a person to be in a non-marriage relationship with someone of the same gender? That also affects society, in making it appear that gays are acceptable. The torah doesn’t say anything about gay marriage only about being gay.
In fact, why not always vote for the Jewish candidate over a non-Jew? If the non-Jew wins it will show our kids that being non Jewish may be ok (as rabbis still meet with him and urge people to vote for him). If your response to this last question is that we live in a country where we don’t have the luxury of being in the majority and we have to live with the majority’s values and religions as part of “society”, then why should gay marriage be any different?