Good evening, Curious Cat. I'm not convinced you are being entirely sincere in your request for a logical argument opposing same sex marriage since your question is framed with references to "religious drivel" and your own assertion to the "right side" wanting to "pick and choose what government controls." It seems you may have already made up your mind. However, I will oblige you with a logical argument against permitting gay marriage that may or may not seem reasonable to you.
The first question we need to answer is this: "Does society have the right to define which parties are permitted to marry?"
I posit to you that the answer is "yes" -- and I further posit that you agree with me. That is, unless you think that people should be able to marry their siblings, or you think multiple spouses should be allowed, or maybe, just maybe, you think someone with a harmless animal fetish should have the ability to court the barnyard fowl of his dreams and experience the dignity and beauty of state-sanctioned matrimony.
So, now that we have established that society has a right to define which parties are permitted to marry, and you wholeheartedly agree with me, you and I have some work to do. We have to decide who gets the honeymoon suite and who ends up sneaking into the henhouse under cover of darkness.
Homosexuality has been taboo for a long time in our society, and until only recently the gay folks would have been hiding with the dude in the henhouse. I am not saying that the two are equivalent -- hens obviously lack the cognitive capacity to love or even to sign the documents necessary to complete the arrangement -- but what I am saying is that attitudes toward homosexuality have changed dramatically in the past 20 years or so.
We're at the point where we don't think homosexuality is a big deal any more and so we say "eff it, let them get married." Even so, we are still as bigoted and narrow-minded as we ever have been. I'm guessing we'll never let a sister marry her brother or let an 8 year old marry a 60 year old in my lifetime, even though such bigotry would never stand the test of the court. There is simply no good defense for such bigotry other than the assertion that we have the right to be bigoted or -- if you prefer -- to establish morals, codes of conduct, etc. that society values and rewards with "special privileges" like marriage.
So, there you have it. I hope you feel enlightened now that you have expanded your mind by opening it to an alternative perspective based purely on logic.