Why shouldn't two women get married?
1. Eve was created as a "helper" for Adam, and child-bearing came into the picture only after they disobeyed God's command by eating the forbidden fruit and were thrown out of the Garden of Eden. God had offered Adam any of the other animals he might choose to be his helper, but Adam didn't like any of them, so God created Eve. You can read about this in the Bible, Book of Genesis: Genesis chapter 1 contains a brief summary account of the creation of Adam and Eve, but the longer account in Genesis chapter 2 gives the details.
Now if the main purpose of Eve was to be a helper for Adam, and if God was willing to let Adam have an animal as his helper, why shouldn't a man have a man as helper, or a woman have a woman as helper?
2. Sex between women is nowhere called a sin in the Bible. The only derogatory mention of it is as being "shameful" in St. Paul's Epistle to the Romans, but shame is not equal to sin. (That's according to the original Greek, but some translators have distorted the text to further their theological agendas -- for example where the Greek word "desires" appears, some translators have added "sinful desires.")
3. The word "wife" doesn't appear anywhere in the Bible in the original languages, not in the Greek New Testament, nor in the Hebrew Old Testament: only the word "woman" is used in both languages, and, once again, wherever you see "wife," it is a translator imposing his or her theological agenda on the text.
4. There is no institution of marriage or a marriage ceremony in the Bible, only wedding feasts are mentioned. The famous passage in Genesis about a man and a woman becoming "one flesh" refers to the sex act, not to marriage. This is shown by the fact that St. Paul, in his Epistle to the Corinthians, chapter 6, quotes that Genesis passage in describing a man having sex with a prostitute.
5. The religion of the Bible is polygamy, not monogamy. In the Old Testament, for example, in the book of Deuteronomy is a passage regulating the inheritance of a man's estate in a case where he has more than one wife. In the Book of Judges, the story of Gibeah, an incident involving a man's "concubine" is related, and the man is a Levite, of the priestly tribe. This suggests he had more than one "wife."
There's only one place in the New Testament that mentions monogamy, but that is qualified by referring specifically to church officials. Monogamy is a pagan practice added to Christianity from the Romans and Greeks. (I once asked an Orthodox Jewish rabbi about this, and he agreed that the Old Testament allows polygamy.)
