The purpose of the evolution of two separate genders from our very distant roots of asexual reproduction, "budding" if you will, was to reproduce the species in such a manner as to foster a greater genetic diversity and thus survival of the species. The AI industry has found that mammalian sexuality is malleable to such an extent via associative conditioning, that if you take a normal male animal and present him with estrus cues such as an estrus female present nearby or a rag soaked with her estrus urine, you can "train" him to release onto other male animals, a dummy mount and so on. In no time at all he will come to prefer the sights, smells and sounds associated with the new "passion-object" and not even an estrus female presented right in front of him to mount. Yes, it's true.
Taken forward, comparative pschology, championed by people like Darwin, Pavlov and Skinner, to name the most prominent of scores, we can extrapolate that because we are also, factually, mammals, we too are subject to the same ability to be "trained" via our first sexual experiences to only prefer a certain set of stimuli from then on.
So the gay argument holds some water in that they are partly right, they cannot be retrained. With the AI industry careful steps are taken to not "spoil' a good stud animal by allowing him free access to females before he is trained. It is better if their drives are first frustrated a little by no access and then the only "normal" sex they can experience is that which is foisted upon them by environmental factors...in most cases their human handlers and a very specific regime set up for the collection of semen in a safe environment. Once trained, they are like push-button...very easy to get to climax by just the sounds and smells of the "first" associatve environment..
Another thing human psychology and comparative anthropology teaches us is that humans are social-learners. Probably moreso than any other animal species. Now, add that social-contagion factor to the fact that sexual-preference can be conditioned by environmental pressures.
And then read the article
Conditioning and Sexual Behavior: A Review again:
http://www-psychology.concordia.ca/fac/pfaus/Pfaus-Kippin-Centeno(2001).pdf
If deviant sexuality becomes mainstreamed, we can deduce, logically, that the numbers of deviants within a given population will rise. The frog will slowly boil in the pan but not from what is originally thought: a process of higher reasoning. Instead, that frog will slowly boil in the pan because over generations we can predict that deviant sexuals within our population [and therefore empathy towards themselves essentially] will rise, via associative conditioning and environmental pressures, to such numbers that the way Prop 8 tried to define "normalcy" will indeed be seen as "abnormal"' because by then it will be...
...Higher reasoning has very little to do with socially-manipulating powerful and malleable sexual drives..
My last point is that not everyone opposed to gay marriage is a christian zealot. How many christians you know believe in comparative psychology and evolution? Yes, that's right. Thinking people also object. However, no matter what your background, you will be hounded being called "homophobe" and so on, and witch-hunted like the beauty-contest girl for having a lucid opinion that opposes the gay tidal wave of special-interest pressure.