"First, if your justification of state involvement in marriage is the production and protection of children, then I think you open yourself to intervention of the state beyond what a limited government conservative should be comfortable with. If protection of marriage by the state for such a goal is the standard, many other activities should be outlawed. Adultery, divorce, pornography are all things that create a poor environment to raise and nurture children, but I don't see us banning said actions."
Conservatives are committed to limited government, and I'm a conservative. It is obvious, I hope, that the state ought not be involved in every form of human association. State involvement in any particular type of human association must therefore be justified. We want as much government as we need, but no more. The state is coercive by its very nature, as it must be if it is to be able to enforce its mandates and exercise its legitimate functions, and is therefore at odds with the liberty and autonomy of citizens. It is not obvious that the government should be in the marriage business at all. The burden is on the state to justify its intervention and regulation. But there is a reason for the state to be involved. The state has a legitimate interest in its own perpetuation and maintenance via the production of children, their socializing, their protection, and their transformation into productive citizens who will contribute to the common good. (My use of 'the state' needn't involve an illict hypostatization.) It is this interest that justifies the state's recognition and regulation of marriage as a union of exactly one man and exactly one woman.
If one takes this view, does it follow that adultery, divorce, and pornography should be outlawed? Not at all. Slippery slope arguments are one and all invalid. (Side-issues I won't pursue: (i) Adultery is a legitimate ground for divorce, so divorce cannot be outlawed. (ii) Another freason why divorce ought not be outlawed is that it is often good for offspring.)
Slippery Slope Arguments
But perhaps I should say something about slippery slope arguments. They come up quite often, in the gun debate, for example. "If citizens are allowed to own semi-automatic pistols and rifles, then they must be allowed to own other sorts of weaponry." That is often heard.
There is, however, no logical necessity that if you allow citizens to own semi-automatic rifles, then you must also allow them to own machine guns, grenade launchers, chemical and biological weapons, tactical nukes . . . . At some point a line is drawn. We draw lines all the time. Time was when the voting age was 21. Those were the times when, in the words of Barry McGuire, "You're old enough to kill, but not for votin'." The voting age is now 18. If anyone at the time had argued that reducing the age to 18 would logically necessitate its being reduced to 17, then 16, and then 15, and so on unto the enfranchisement of infants and the prenatal, that would have been dismissed as a silly argument.
If the above anti-gun slippery slope argument were valid, then the following pro-gun argument would be valid: "If the government has the right to ban civilian possession of fully automatic rifles, then it has the right to ban semi-automatic rifles, semi-autos generally, revolvers, single-shot derringers, BB guns, . . . . But it has no right to ban semi-autos, and so on. Ergo, etc.
I have been speaking of the 'logical' slippery slope. Every such argument is invalid. But there is also the 'causal' or 'probabilistic' slippery slope. Some of these have merit, some don't. One must look at the individual cases.
Supposing all semi-auto weapons (pistols, rifles, and shotguns) to be banned, would this 'lead to' or 'pave the way for' the banning of revolvers and handguns generally? 'Lead to' is a vague phrase. It might be taken to mean 'raise the probability of' or 'make it more likely that.' Slippery slope arguments of this sort in some cases have merit. If all semi-auto rifles are banned, then the liberals will be emboldened and will try to take the next step, the banning of semi-auto pistols. The probability of that happening is very high. I would lay serious money on the proposition that Dianne Feinstein of San Bancisco, who refuses to use correct gun terminology, though she knows it, referring to semi-automatic long guns as 'assault rifles,' a phrase at once devoid of definite meaning and emotive, would press to have all semi-autos banned if she could get a ban on semi-auto rifles.
But how high is the probability of the slide in the other direction? Not high at all. In fact very low, closing in on zero. How many conservatives are agitating the right to buy (without special permits and fees) machine guns (fully automatic weapons)? None that I know of. How many conservatives are agitating for the right to keep and bear tactical nukes?
I return to my reader's claim. He said in effect that if the State regulates marriage then we are on a slippery slope toward the regulation and in some cases banning of all sorts of things that are harmful to children. But the argument is invalid if intended as a logical slippery slope (since all such arguments are invalid), and inductively extremely weak if intended as a causal or probabilistic slippery slope. The likelihood of, say, a clamp-down on the deleterious dreck emanating from our mass media outlets is extremely low.