> in which a person asserts that some event must inevitably follow from another without any rational argument or demonstrable mechanism for the inevitability of the event in question.

His argument is not a fallacious one. The reasoning used to justify applies to other scenarios. Polygamy is the main one brought up on this thread: Why are we telling adults who and how many people they can marry?

If you legalize [x] because [y] and [y] also applies to [z] then [x] and [z] are equivalent for reasons [y]; therefore [z] should be legalized if [x] is legalized unless [x] and [z] are demonstrably not equivalent for reasons [y].

I'd also like to note that the cutoff for being an adult should be 25, not 18. Therefore people shouldn't be able to be married until they are 25. The age of 18 is arbitrary and in many places that age is 16. In some places that age is 14. When exactly is the cutoff? Well it's based on an individuals mental maturity, which is not equivalent to their age. I've met 12 year olds who are more rational and mature than 30 year olds.

Furthermore:

Hebephilia and pedophilia are often merged concepts when people speak. For the people speaking on this subject, can you clarify which you're talking about? It's difficult to tell which people mean, but it usually gets clarified to the 13-16 year old ranges. At that age, I'd argue most children understand marriage, can be educated on the government-related issues to marriage, and understand sexual relations and pregnancy. The idea that they can consent to sexual relations of other minors, but not of adults, is some form of double-think. Note that in this context it doesn't seem marriage is what is being brought into question: just sexuality.

Note that I'm not defending it: I think people shouldn't be able to have sex or get married until at least age 25 unless they can demonstrate the ability to proper care and provide for a child with a stable income.

Can you name one other kind of marriage that is demonstrably more likely to get legalized because we legalized same-sex marriage?

If there's any kind of slippery slope, it's the slippery slope from man-woman marriage to adult-adult marriage. Well, we've slid down it, and now we're at the bottom.

We haven't slid to "adult-adult marriage" yet until the above are legalized across the board. So please don't misrepresent man-man and woman-woman as being adult-adult (or adult - n adult)

Since marriage does not imply reproduction, an incestual couple can refrain from having kids, adopt, use a surrogate/donor, etc. so there is no "but imbreeding" argument. Which is an argument about genetics... Isn't choosing who can reproduce a form of eugenics? We don't stop people with known hereditary complications from choosing to have kids, why should we stop siblings? If they choose to accept the risks, why should society have any say in the matter?

Some could argue a failed relationship can lead to issues in a family household but I don't think that has any real merit. Dating a family member's best friend and it going sour can have similar consequences.