Dude, it’s a contract. Read it before you sign it. If you don’t like it or don’t understand it, don’t sign it. It’s really not that complicated.
Suppose a rental car company states that it costs $40/day for both members of a married couple to rent one car, and $45/day for two non-married individuals to rent and drive one car. Suppose further that two people arrive at the counter and show the clerk a contract that specifies that neither of them shall engage in any sexual activity, except with the other, without providing advance notice of such intention, and that violation of such terms shall entitle the aggrieved party to compensation of $10,000 payable at 10% interest as of the date of a discovered infraction. Should those two individuals be charged $40 or $45 for the rental? On what basis should the clerk make the determination?
A major point which conservatives miss (I'd argue that it's the most important point) is that many individuals, institutions, and companies voluntarily give certain benefits exclusively to married couples, on the expectation that they will indirectly benefit from doing so. Those pushing gay marriage don't like the fact that many of the institutions, companies, and even individuals would want to offer such benefits to people in "traditional" marriages, but not offer them to people in "non-traditional" marriages. If a question "Is X married to Y" can be quickly, easily, and unambiguously answered yes or no, then individuals, institutions, and companies that wish to voluntarily offer benefits exclusively to married couples will have a practical means of doing so. If the state is not involved with marriage, then the question "Is X married to Y" may become essentially unanswerable unless one is willing to regard as "married" any pair of individuals who claim to be married to each other.