I can see no good reason to exclude same sex couples from all the same legal privileges of marriage as everyone else. Moreover, I can see of no good reason why this should have any legal effect on a personal freedom of religion. I know that right away some will read the above passage and think that there are indeed reasons to dispute my opinion. But please keep in mind that I wrote that there were no GOOD reasons, not that someone couldn't come up with something.
I do understand that for a vocal group of Christians, the issue of homosexuality is a deeply moral one. To them it is a choice to engage in a set of behaviors that are seen as sinful, perhaps even evil. Justifications for this view can be found in their holy book, so they do have theological grounds for their objection to homosexuality.
Often Christians like these also hold as a part of their expression of their religion the need to remake the society around them into one that is in line with their theology. Proselytizing is an integral component of their religion as it is done to help make mankind more like them and hence more beloved by God. Laws or policies that cannot be justified theologically or are in contrary to their theology are opposed. Laws or policies that are inspired by their religion are always promoted.
The problem with legislating from a theology is that it violates the Constitution as it effectively forces behaviors of a religious origin on non-believers or practitioners of another faith. I do understand that there are religious rules that also appear in the laws of our country, such as don't murder. However, those laws exist for perfectly reasonable reasons that don't require a theological basis. They can and must be justified in secular terms in order to be Constitutional.
The freedom of religion in our Constitution is absolute in terms of belief, meaning that we are free to believe as our conscience dictates. What the Constitutional freedom of religion does not protect is all conduct that might be associated with the expression of a religious faith. For example, a religion might require its faithful to cut the beating hearts out of human slaves every morning -- but that would rightly be considered murder under the law and not protected under the First Amendment.
Christians do have the legal right to believe that all homosexuals are sinful abominations who will burn in hell for all eternity. They also have the legal right to believe that homosexuals should be stoned to death outside the city walls. Regardless of those beliefs, Christians are held to the same standard of conduct under the law as everyone else and aren't allowed to murder or force other citizens into second class roles as an expression of their faith.
So if we take the theological objections out of the equation of the LEGAL rights of people to get married in this country, what is left?