I have written before here that I think the state should get out of the marriage game entirely. If people want to get married (within logical parameters), let them go to their preacher, rabbi, pastor, Imam, whatever and get married in the eyes of thier God. None of the State's business. None of my business, frankly. If people are getting married in strange churches whose teachings I don't accept, I can decide that I personally don't respect the "marriage" of those two people (or ten people, or people and houseplants, whatever). The point is, it's a personal opinion whether or not I want to respect or honor another's marriage (and whether they want to honor my marriage).
The problem with gay marriage as it's being pushed right now is that by getting the STATE to sanction the gay marriages, I no longer have the personal right (legally) to disrespect or dishonor a marriage whose legitimacy I disagree with.
And most importantly - here's the part I just figured out (maybe I'm slow) - the STATE, by honoring these marriages, is taking the OFFICIAL, GOVERMENT position that my religious faith is WRONG, that the teachings of my church are IN ERROR. That's what offends me. Isn't it the liberals who are always shouting about a separation between church and state? Well, these liberals are asking the Government to take an official position that it explicitly disagrees with my faith and my Church. It has singled out for favorable treatment those "religions" that hold a particular viewpoint on an issue of faith, and has disrespected as a matter of policy, my faith.