To distinguish themselves from other “Christian” bodies that support it. People need to know to make a wise choice as to where they are going to worship and learn.
Sure has put the Southern Baptists at odds with many of the
“mainstream” Protestant churches. Pretty effective recruiting strategy, as well as being on the right side of the theological question regarding marriage.
Whatever “gay marriage” may be, it is not a Christian sacrament. If you have even the least acquaintance with the various works of the Marquis de Sade, one of the themes was the desecration of all things religious, and a celebration of unnatural unions.
To get away from being lumped in with Joel Osteen, Rick Warren, Methodists, ECA, and the likes.