The argument against homosexual marriage is simple, homosexuality is abnormal (whether by birth or upbringing is immaterial) and to reward it with complete equality to heterosexuality is absurd on its face.
So why is it abnormal? Simple. There is one characteristic common to all life, the ability and drive to procreate, to survive (there are no life forms that are immortal). Homosexuality by definition is a biological dead end. As a corollary, being born blind, or becoming blind by other means, is abnormal. How do we know?...because nature gave us eyes so we can see. One can argue both are disabilities. Should we discriminate against that blind person?..while most people would say we shouldn’t, we do it all the time...we don’t issue them drivers licenses, allow them to operate helicopters, perform brain surgery, and countless other things.
Some might say, homosexuality is present in only 2-3% of the population, so should it really matter...but homosexuality is still abnormal, regardless if there was only one or a billion, nor does it matter if its caused at birth or comes about after birth, just like with a blind person.
So, should we have unlimited discrimination against people who are homosexual? In most respects, absolutely not (just like with the blind, and others with disabilities)...but I view marriage as an exception...Since there is no scientific evidence of a homosexual gene (after a lot of searching), there is the very likely possibility that homosexuality results from post birth factors...by allowing homosexuals to marry and raise kids we may in fact be contributing to a sexuality that is proven to be abnormal.
Another away to make my point. Assume God like powers for a minute. You have to decide one issue - the nature of life throughout the universe, and you have 3 choices. A) all life is heterosexual, B) all life is homosexual c) flip a coin, head: heterosexual; tails: homosexual. If you believe homosexuality is just an alternate life style, and equal to
heterosexuality, the right choice is C - although there is a 50% chance you will extinguish all life in the universe. Of course, the only logical choice is A.