Every once in a while, I read an article about some congregationalist church in the south that refuses to marry interracial couples or something. They are allowed to discriminate in that way if they do so on religious grounds in the context of a religious ceremony, but fortunately the government and non-religious businesses that are considered public accommodations, like restaurants, are not allowed to discriminate against interracial couples.
That's the same way gay marriage is likely going to go- churches don't have to marry gays if they don't want to, but gays will have the right to go to a court house and get civil marriages with all the legal and taxation rights that come with it, on the grounds of equal protection under the law. They'll also be allowed to buy wedding cakes from businesses that sell cakes.
If all these years after desegregation, churches can still decline to marry interracial couples, I think it stands to reason that churches will still be able to decline to marry gay couples if they want to. Of course, some churches won't want to decline to marry gay couples (i.e. Episcopalians are working on a proposal to take gender distinctions out of their wedding ceremonies), and they'll be allowed to marry gay couples if they want to.
Churches are allowed a lot of leeway on who they choose to allow to marry and who they don't. For example, I went to school with a guy who was raised, baptized, and confirmed Roman Catholic and went to mass frequently. His family even had priests over for dinner and the like sometimes. He got engaged to a Catholic woman of about the same age. The pastor at the local parish refused to marry them, citing psychological immaturity, which he was legally allowed to do even though they met all the legal requirements for marriage.
So, the couple in question decided to meet with a Methodist pastor. The Methodist pastor agreed to marry them (Though he didn't have to either), and they got married in a Methodist church building. If the Methodist pastor had declined, they could have checked with other ministers from other denominations, or just gone to a court house and gotten married by a justice of the peace. I don't think the church they grew up in recognized their wedding, which I thought was kind of a shame, but the point is that we already have plenty of precedent of churches being allowed to not marry people for religious or personal reasons even if they meet the legal requirements, and not having to recognize a civil marriage or a marriage in another church, denomination, or faith for their own religious purposes.
As far as people saying that those who oppose gay marriage won't be allowed to speak about it, I think those fears are unfounded. Why? Well, again, even though discrimination against racial minorities is now illegal, people are still free to express racist views, and fly Confederate flags. It's not really socially acceptable to be racist, but the constitution doesn't guarantee you the right to have your views socially accepted, it just gives you freedom of speech, freedom of religion, and freedom from discrimination in the workplace and in public places like restaurants and such.
So, if people's fear is that they might be looked down on for opposing gay marriage, then, yeah that might happen. So what? Are you guys advocating that the government protect you from people looking down on you?