That's a pretty strong statement. Since you obviously don't oppose gay marriage you don't know this first hand. How have you aquired this knowledge of motivation? I'm highly skeptical of it really. I'm not sure what the common motivations are, to be honest, but I'd need some serious evidense to convince me of your assertion.
Aaron
This is the motivation behind all homophobia. It is a rather complex subject and I am writing a book on it (not just applied to homophobia, though).
You will arrive at the same conclusion if you think about the issue long enough. The evidence is quite simple:
1) Gay marriage would not in any way affect traditional marriage except in the way people view the institution of marriage.
2) Such an effect would logically either be positive or negative.
3) If the effect was to be positive, nobody would oppose it.
4) Therefore the effect is presumed to be negative by opponents of gay marriage.
5) All but one function of traditional marriage that could possibly be undermined by gay marriage can already be undermined by traditional marriage (such as couples professing how much they love each other, raising children, etc). In other words, gay marriage could NOT logically harm these functions of traditional marriage.
6) The remaining function of traditional marriage is to separate those that are married from those that are not -- giving benefits to those that are married. This fact is indisputable.
7) This last function CAN be harmed by allowing gay marriage, because it lets more people ("fags" and "homos" and "dykes", to boot!) into the elite group of "married couples."
8) Therefore, anyone opposing gay marriage must be opposing it for the reason I put forth. Or at least, anyone who isn't a moron and opposes it, because only morons would argue that the functions in point 5) actually would be undermined by gay marriage.