The argument I hear almost everytime from Christians who are opposed to Gay Marriage, Pro DADT, or just opposed to Gay rights in general is that the bible says its wrong.
That is true
Leviticus
20:13 If a man lies with a man as one lies with a woman, both of them have done what is detestable. They must be put to death; their blood will be on their own heads.
Leviticus also has some other interesting laws.
Don't let cattle graze with other kinds of Cattle (Leviticus 19:19)
Don't have a variety of crops on the same field. (Leviticus 19:19)
Don't wear clothes made of more than one fabric (Leviticus 19:19)
Don't cut your hair nor shave. (Leviticus 19:27)
Any person who curseth his mother or father, must be killed. (Leviticus 20:9)
If a man cheats on his wife, or vise versa, both the man and the woman must die. (Leviticus 20:10). I wonder if Dr. Laura would like that one to be enforced?
If a man sleeps with his father's wife... both him and his father's wife is to be put to death. (Leviticus 20:11)
If a man has sex with a woman on her period, they are both to be "cut off from their people" (Leviticus 20:18)
Psychics, wizards, and so on are to be stoned to death. (Leviticus 20:27)
If a priest's daughter is a whore, she is to be burnt at the stake. (Leviticus 21:9)
People who have flat noses, or is blind or lame, cannot go to an altar of God (Leviticus 21:17-18)
Anyone who curses or blasphemes God, should be stoned to death by the community. (Leviticus 24:14-16)
Don't let cattle graze with other kinds of Cattle (Leviticus 19:19)
Another argument is that the bible defines acceptable relationships as the Marriage of One Man One Woman.
Genesis 4:19 And Lamech took unto him two wives.
Genesis 16:1-4 Now Sarai Abram's wife bare him no children: and she had an handmaid, an Egyptian, whose name was Hagar. And Sarai said unto Abram, Behold now, the LORD hath restrained me from bearing: I pray thee, go in unto my maid; it may be that I may obtain children by her. And Abram hearkened to the voice of Sarai. And Sarai ... gave her to her husband Abram to be his wife. And he went in unto Hagar, and she conceived.
Genesis 25:6 But unto the sons of the concubines, which Abraham had....
Genesis 26:34 Esau ... took to wife Judith the daughter of Beeri the Hittite, and Bashemath the daughter of Elon the Hittite.
Genesis 31:17 Then Jacob rose up, and set ... his wives upon camels.
Exodus 21:10 If he take him another wife....
Deuteronomy 21:15 If a man have two wives, one beloved, and another hated....
Judges 8:30 And Gideon had threescore and ten sons of his body begotten: for he had many wives.
1 Samuel 1:1-2 Elkanah ... had two wives; the name of the one was Hannah, and the name of the other Peninnah.
2 Samuel 12:7-8 Thus saith the LORD God of Israel ... I gave thee ... thy master's wives....
1 Kings 11:2-3 Solomon ... had seven hundred wives ... and three hundred concubines.
1 Chronicles 4:5 And Ashur the father of Tekoa had two wives, Helah and Naarah.
2 Chronicles 11:21 Rehoboam ... took eighteen wives, and threescore concubines.
2 Chronicles 13:21 But Abijah waxed mighty, and married fourteen wives....
2 Chronicles 24:3 Jehoiada took for him two wives.... Polygamy was considered MORAL and Normal throughout those times.
What is considered Moral and Natural by society changes constantly.