I think the fundamental argument is mostly based on religion and cultural "norms" across the board. As far as I know (granted, I'm and admitted simpleton idiot), this is the first time in recorded history that being gay is so widely and openly accepted. Not just from the "Christian" point of view, but it seems MOST societies in history have put a taboo on same sex relations. For better or worse, I guess one could say. These types of "norms" in society don't change easily without a lot of resistance from the traditional points of view, I suppose... It threatens long held belief systems. Rightly so as it is opposite of the past way(s) of thinking. This does not necessarily prove to be "logical", per say, but it has been the "norm" for many thousands of years. At least from a societal acceptance point of view.(IMHO)
actually, homosexuality has often existed in history sans opprobrium - in much of ancient greece, for example, it was an accepted practice, no doubt in part given the tremendous population pressures there (many people, not much food)