I really can't say honestly how any of those things are worse than homosexual "marriage." Seriously. It's not a slippery slope argument. Once you allow that homosexuals can be "married" you're already at the bottom of the slope...
At the bottom of the whole argument is the notion propagated by the proponents of homosexual "marriage" that homosexuality is equal to heterosexuality. By their standards, both are normal and equal. The fact that only about two or three percent of humans are homosexual does not cause the proponents to wonder one bit.
The main objective or function of sex is to reproduce. Certain bodily orifices were not designed for entry by sexual organs as the main avenue of sex. If homosexuality were the same as heterosexuality, then homosexuals would be able to reproduce with each other.
And lastly, if further scientific studies prove that homosexuality is a result of overproduction of hormones in the womb (and can be treated), then how can something that would be the result of a bodily malfunction be considered normal or equal?