Take for example limiting NAMBLA. Yes, in many ways the organization is a primer for potential illegal activity. However, having the potential for that and actually doing the illegal act is different. You speak of using "common sense". Here is the issue...there is no such thing as common sense. Such a concept is simply at best a broad collective understanding by a majority of the public. Protecting the individual against the mob mentality of the public was something the founders were absolutely interested in. Additionally, "Common sense" is an unmeasurable, verifiable, or standarized notion. What makes perfect common sense to you could seem backwards to another. Look in regards to most contentious fights...how many people think its "common sense" that we would have an effect on the environment while the other side suggest its "common sense" that we couldn't affect something so large. Think about those claiming that its "common sense" that if we disallow guns on the street that less people will be shot with guns. And on and on.
Take the KKK. Lets say we disallow them to march because its "common sense" they're simply screaming hate speech. What you do is set a precedent for the government to deny freedom of speech due to what's viewed as "hate speech" by an individuals judge of what's "common sense". And yet how many times do we see arguments and back and forths about the "hate speech" of some people in the media for things they say that one side believes its "common sense" to relaize that the persons statements are racist or inciting violence?
Unless you can fully trust the government, or at least substantially trust them, to act in a fair, unbiased, non-self serving, and in our best interested then vesting them the power to deny constitutional rights due to claims of "Common Sense" presents significant pitfalls and issues imho with regards to the expansion of governmental power.
Now you could be here saying "But Zyphlin, you just argued AGAINST The slippery slope fallacy". Indeed I did, however not because I don't think the Slippery Slope fallacy is valid. It is a legitimate and worth while argument or insight so suggest that there is a potential, the degree of which varies per each topic, that if A happens it could lead to B which could lead to C. There's nothing wrong with that. However, the issue comes when one suggests that the slippery slope ALONE is worth while reason to not do something without any other significant supporting or abutting reasons. And even with those things, its a judgement call on each person whether or not the POTENTIAL for the slippery slope outweighs the benefit of the initial actoin.
In my eyes, with regards to marriage, I view the worst case scenario. Lets say somehow, someway, all the extremely paranoid and unlikely cries of those who use the slippery slope argument actually came true. That we end up legally allowing people to **** their dogs, for children to be allowed to consent to sex, and that multiple people can get married. While I may not like any of those three things, issue one and three there has little true and honest affect on my individual freedsoms or life and has few ways that it actually could direclty affect it. The second one I'd have larger issue with, as there are definitely kids of a certain age that I feel should not be able to consent, but even there if I was being 100% honest I would suggest that in our society at times our age of consent laws are rather ridiculous and arbirtray as well. Not to a point that I feel we significantly need to work to change them, but not to a point where I'd have much of a fuss to a certain point. Still however, in such a case, the direct effect on me and my personal liberties is negligable. So all told, even if the most extreme of fears occur, its direct and noticable effect on my life and my liberty would be relatively small. It is not bestowing additional power unto the government but rather instill additional power into the people. Power I may not approve of, but it is at the least not expanding the governments control OVER us.
Conversely, limiting speech or other constitutionally protected rights due to "common sense", when taken to the extreme, allows the government to forgo the constitutional rights for any reason that the party presently in power would deem as "Common Sense". Freedom of speech, religion, and assembly...the right to bear arms...all our rights protecting us from government search, seizure, and illegal enforcement of the law...could be wiped away if deemed "common sense". Take most of the controversial topics of today...Global Warming? Well its common sense that we need people to drive cars that don't emit as much of a carbon footprint so we need to outlaw SUV's. Internet Pirating? Well its common sense we can't have people pirating on the internet so we need to have the power to shut down websites whenever needed. By giving such a broad, generalized, and low floored requirement such as common sense you provide the government with precedent to trample constitutional rights. In the worst case, extreme scenario of this slippery slope my rights as an individual and the rights of society are significantly infringed upon. Our freedoms are reduced and my life is more likely directly affected.
Additionally, we have one slippery slope has a historical trend of occuring in a rather steady and substantial motion. The government has been steadily expanding, finding loopholes and exploits regarding how to function within hte constitution, and seeking to gain more and more control over individuals for decades if not centuries. They have shown time and time again that as you allow the government to do more that they will take that and build upon it (From a Democratic side, look at entitlements from the days of FDR to now. For a Republican side, look at survelliance from the days of the TITLE 3 in the 1968 Crime & Safe Streets Act to PATRIOT). On the flip side, its been over 50 years since the last major change to marriage occured on a national level. Fifty years and we've still yet to see the next shift ACTUALLY occur. To assume that it'd take over half a century to go from Interracial to Same Sex, but that we'd go Same Sex to ****ing animals in short order is basically stating baseless fear. Not to mention, as fearful as they likely were of gays as many may've been in 1967, even then I would imagine there was a larger stigma upon those trying to screw a 7 year old or those trying to have sex with a horse then simply on gay people (indeed, there's a reason why those things are applied stereotypically to gays by some people...specifically to make them seem WORSE, because those things are viewed more negatively than homosexuality itself). With even greater public disdain than homosexuals have, its more reasonable to suggest it would take even longer for the next step of the slippery slope to occur...IF it was going to occur...then it has taken to reach the point of Same Sex Marriage.
So considering the slippery slope's effect on myself and my personal liberties, the powers it would or wouldn't give the government, and the historical and social liklihood of said slippery slope coming to pass, the "Same sex marrige" version of it doesn't pass muster with me. However, giving the ability of government to limit constitutional rights due to "common sense" absolutely does.
While I understand the desire and the want to do such...trust me, I do...ultimately a broader view must be taken. The easy way out is to attempt to simply give the government more power and say "please, deal with these undesirables so I don't have to think about them anymore". However, that kind of notion is present throughout the world and there are many places you could go to live under such an idea. To me, what makes America special and different...and what it needs to hold onto...is that just because it may be easier to have the government do that it instead puts the responsability and power upon the individual to deal with it in some legal way, even if that way is simply ignoring it, so as to preserve the ability of that same individual to act freely and on his own accord in other aspects of his life as well.