However, for those wondering, What are they thinking?, several anti-same-sex-marriage groups have produced apologies that tell exactly that. Everyone, pro and anti, should read the arguments.
I suggest the Family Institute of Connecticuts tract as a start. It covers a lot of ground and lays out the talking points. For example:
Apart from the social cost, there will be substantial financial cost. In fact, much of the literature in favor of same-sex civil unions is about money and benefits. One expense resulting from the Vermont legislation was requiring insurance companies to insure civil union partners for medical insurance policies and requiring business owners to pay for medical insurance coverage for these partners equivalent to that offered married couples. Business owners would not be free to pay just for married couples, even if, as a matter of conscience, they do not support same-sex unions.Note: The link above opens an Adobe Acrobat PDF. You need a compatible reader for it.
It is almost unthinkable for a man and a woman to marry merely to receive medical benefits, or any other benefits, for that matter. Parties to a civil union, on the other hand, facing potentially enormous health costs, will be tempted to enter into civil unions in order to move the cost of their health coverage from themselves to a business owner.
....If same-sex unions become equivalent to marriage under the law, then we should expect that those who disagree with the model of monogamous marriage will move to attack that model.