I'm not going to delve into the whole question of whether the Bible is across-the-boards anti-gay. The question in this particular case isn't "Is homosexuality absolutely, 100% wrong in all cases according to the Bible?" but rather "Should the state be making laws based on the ethics and morality of a limited number of religious traditions, even if one of those traditions is mine?"
Homosexuality is frowned upon in the Abrahamic traditions-- Christianity, Judaism, and Islam-- and in those religious traditions that put a great emphasis on chastity, procreation, or the limiting of sensual pleasures (though historic texts would indicate it hasn't always been frowned upon in any of these). Other religions are often much more open to homosexuality and many contain stories, myths, traditions, and rules on gay relationships. The ban on gay marriage is basically imposing the Abrahamic traditions' morality on all marriages.
Would you support an amendment to the Indiana state constitution that said that only marriages performed by an Islamic Imam, blessed by Allah of the Quran, and performed in a mosque or other site sacred to Moslems would be recognised by the state as valid? How about performed by a priest and priestess of Isis, in a sacred grove, under the full moon while skyclad (that's nude for those who don't know)? How about those performed by Christian clergymen, blessed by the God of the Bible, and performed in a church? That might seem great on the face of it, but is it really? What about if the state specified that NO religious marriage would be recognised, but only those performed by a secular authority, in a secular setting, with only secular vows, music, and readings?
More and more, I've come to the belief that the state really shouldn't be authorising marriage at all. For Christians, we place our marriages in God's hands to bless, sanctify, and uphold, not the government. I can guarantee if the last case in the paragraph above ever came to pass, there would still be marriages being performed in the church, even if the state didn't recognise them.
"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof." -- First Amendment of the US Constitution.
"No law shall, in any case whatever, control the free exercise and enjoyment of religious opinions, or interfere with the rights of conscience." -- Article 1, Section 3 of the Indiana State Constitution.
"No preference shall be given, by law, to any creed, religious society, or mode of worship; and no person shall be compelled to attend, erect, or support, any place of worship, or to maintain any ministry, against his consent." -- Article 1, Section 4 of the Indiana State Constitution.
Limiting the nature of marriage to only that recognised within a limited number of religious traditions is in opposition to the freedom of religion that we exercise within this state.
By allowing this, we are setting off down a slippery slope where we allow the government to impose moral restrictions based on religious traditions, rather than on the upholding of rights and freedoms. It may not affect you if you happen to be in the majority religion, but what about those who aren't? Is the resentment of being reduced to second-class status likely to result in many genuine conversions or will it drive people away or lead to false conversions? Does this aid in our carrying out of the Great Commandments (Love God and Love Others) and Great Commission (Go into the world and share the good news)? Or, like many other times when we try to make non-Christians behave like Christians, instead of introducing them to the person of Christ so that they can become Christians themselves, is it harmful to our witness?
And what happens when your religion is no longer the majority? The time is coming when it may not be. Go read the Revelation if you don't believe me. Do we want a country that ignores the right to free exercise of religion when that time comes upon us or our children or grandchildren? Take heed, brothers and sisters. Take heed.