Everyone wants to say that they are for "equality." But as Steven Smith points out this becomes this becomes an empty term when applied to marriage as "marriage equality" (http://www.thepublicdiscourse.com/2013/03/7912/). Proponents and opponents of homosexual marriage are really disagreeing about what marriage is. For this fundamental question, the best treatment I have encountered remains: http://surburg.blogspot.com/2013/03/what-is-marriage-what-it-is-why-it.html
Thanks to Andrew Packer to posting the Steven Smith piece.