This excellent analysis was written by Dr. Patrick Lee, professor of theology at Franciscan University of Steubenville:
A recent story in Newsweek claimed that the only reasons for opposing same-sex "marriage" are religious. But there are powerful arguments for marriage rooted not in faith but in reason.
In the December 15th  edition of Newsweek, both Jon Meacham in his editor’s note and religion editor Lisa Miller in her front-page article mock arguments from scripture. At the same time, they invoke that same Bible’s authority for a “more general” message of “inclusivity,” in order to lobby for making gay marriage a sacrament. Meacham and Miller paint all opposition to the radical re-definition of marriage as hateful bigotry, comparing it to racism, and labeling appeals to the authority of the Bible against homosexual “marriage” and homosexual acts as fundamentalism. Indeed Meacham goes further: it is “the worst kind of fundamentalism.” How much worse than suicide-bombings and beheadings he does not make clear.
Others can dissect the theological and factual howlers in these essays. Here I want to correct the assumption made by Meacham and Miller that the case against same-sex “marriage” must be a Biblical one. Instead, both by faith and by reason one can see that genuine marriage must be heterosexual, that sexual acts outside of marriage are immoral, and that the state, therefore, should not declare any same-sex unions “marriages,” nor actively encourage sexual acts outside of marriage.