This comment is so off-base and silly I don’t even know where to begin.
The slippery slope argument is a bad and unconvincing argument. Just argue against Toeiva marriage and/or state regulation of marriage. It has nothing to do with parents marrying their kids, people marrying dogs, etc.
It is not a “slippery slope argument”. It is a fantasic loophole in the tax code that could save families millions of dollars. I don’t expect these marriages to be “real”, but real enough for tax purposes (as the two parties do care for each other, etc.) that the IRS would have a hard time fighting it.