"Do not lie with a male as one lies with a woman; it is an abhorrence. (Lev 18:22)
"But you must keep My laws and My rules, and you must not do any of those abhorrent things...."(Lev 18:26)
"All who do any of those abhorrent things -- such persons shall be cut off from their people..." (Lv 18:29)
So God admonishes his people in the Bible. If you are a true Bible-believer and that the Bible is the word of God - the "word" about homosexuality (just male homosexuality, by the way) is quite clear - God is against it. It is an "abhorrence."
The Bible, however, does not constitute the laws of the United States. Follow it to the letter if you are so inclined, but do not impose it on others. No "sharia" law here. You must respect the legal rights of your fellow citizens.
Homosexuality is genetic - but even if it were not - even if it were a choice - all of us are free to live as we please so long as we cause no harm to others. Gay marriage hurts no one. No government should have the right to limit the values and lifestyles of consenting adults whose activities injure no one. Whom one loves, whom one is intimate with, whom one wishes to marry are not its business.
The Declaration of Independence's endows us with certain "inalienable rights." Freedom of sexual association between consenting adults, including freedom to marry whom you wish is such a right, and though the Declaration is not a system of laws, it is a system expounding the basic principles of freedom upon which this nation was founded.
Legalizing gay marriage will injure no one. Individuals have every right to view it unfavorably, but no rights to legally prevent it. If you share the opinions attributed to God in Leviticus, so be it - but you must agree that only God can decide how gays will be "cut off from their people" - not you. God has no legal representation in this world - maybe in the world to come (as some would believe), but not in these United States!