That and the porno thing are way more about age than ideology. Old democrats are against same-sex marriage and young republicans are more likely to be for it than most other deviations from the party line. Look at a graph of opinion on that issue over age sometime.
I suppose ideology is in there a little as far as secularism (!= atheism), but hey, it's the internet.
And redditors will definitely treat you that way. They usually don't care if your objection is well-reasoned or articulately expressed, they just instantly destroy anything vaguely pro-religion, pro-life, pro-traditional-marriage, etc.
Only amongst those opposed to gay marriage. It is very much a human-rights issue for everyone I know, particularly those who are gay. As for the decisions reddit takes as a site (not the community), they are generally both popular with the community and the admins. Take, for example, the stance they took on running pro-prop 19 ads, or their decision not to run anti-gay marriage ads. The first was genuinely political, but it was also popular. The second was a matter of principle.