The slippery slope argument is always used by those seeking to push their fear based agenda. It really is pathetic, and quite illogical. When they raised the speed limit in America from 55 to 65 (or 70) how many believed the 'slippery slope' argument then? That if we raised the speed limit to 65 the slippery slope meant we would soon be enacting 95 mph speed limits. Or the 'slippery slope' debaters when it comes to assault rifles. When govt seeks to outlaw AK-47s (machine guns) the 'slippery slope' hunters start decrying that what's next a 30/6? How many people really hunt (other than for hunting other humans) using an AK-47? They have nothing in common. It's a fear tactic, and frankly any time someone needs to make an appeal to your emotions, has already lost the argument.
Just a few days ago Mildred Loving died. She was the one responsible for getting the Supreme Court of the United States to strike down anti-mixed race marriage prohibitions. Yes, only 40 years ago getting married to someone of an opposite race meant you ran afoul of the law, and were not welcome in many states. Google Mildred Loving, and read her story. Then tell me that white and black people or mixed races shouldn't marry. Gay marriage is exactly, absolutely, 100%, identicaly to this ruling back in 1967. In 20 years time modern America will look back at this time frame and wonder what kind of uneducated, neanderthal, idiots, used to run the place.