Back when interracial marriage was a contested legal issue, were there any opponents that used the "slippery slope" argument, and claimed that it could lead to legal homosexual marriage?
Not a direct answer, but one reason it was claimed miscongenation was wrong, was because it produced more effeminate off spring.
In court they certainly used the same slippery slope arguments they're using now against gay marriage:- interracial marriage would lead to incest, pedophilia and polygamy, but I'm not aware of any direct slippery slope argument claiming it would lead to gay marriage.
“The underlying factors that constitute justification for laws against miscegenation closely parallel those which sustain the validity of prohibitions against incest and incestuous marriages.”
(Source: Perez v. Lippold, 198 P.2d at 46 (Shenk, J., dissenting, quoting from a prior court case))
“[T]he State's prohibition of interracial marriage . . . stands on the same footing as the prohibition of polygamous marriage, or incestuous marriage, or the prescription of minimum ages at which people may marry, and the prevention of the marriage of people who are mentally incompetent.”
(Source: Excerpted United States Supreme Court oral argument transcripts from Loving v. Virginia, from Peter Irons and Stephanie Guitton, eds., May it Please the Court (1993) at 282-283, quoting Virginia Assistant Attorney General R. D. McIlwaine, arguing for Virginia's ban on interracial marriage)