Justice Scalia put this question to Ted Olson, arguing the case for overturning California’s ban on same-sex marriages, today:
JUSTICE SCALIA: We don’t prescribe law for the future. We — we decide what the law is. I’m curious, when - when did — when did it become unconstitutional to exclude homosexual couples from marriage? 1791? 1868, when the Fourteenth Amendment was adopted? Sometimes — some time after Baker, where we said it didn’t even raise a substantial Federal question? When — when — when did the law become this?
MR. OLSON: When — may I answer this in the form of a rhetorical question? When did it become unconstitutional to prohibit interracial marriages? When did it become unconstitutional to assign children to separate schools.
JUSTICE SCALIA: It’s an easy question, I think, for that one. At — at the time that the Equal Protection Clause was adopted. That’s absolutely true. But don’t give me a question to my question. (Laughter.)
Andrew Sullivan’s answer  is “these forms of discrimination became unconstitutional once the collective consciousness of Americans recognized that the discrimination was unjust,” but Scalia would presumably respond by pointing out that “collective consciousness” is not law. If Olson is arguing his case on equal protection grounds, he should have given Scalia the answer that Scalia gave him: the 14th Amendment. Scalia might then want to see where in the debates over the framing of the 14th Amendment any ban on prohibiting same-sex marriage could be found, but then again, he knows those debates don’t include any express ban on prohibiting interracial marriage, either. The latter case  relies on the principle of the 14th Amendment and the civil rights legislation it was intended to enable, rather than on explicit discussion of marriage in the debates.
Aside from such comparisons, the question of how an intended principle may be applied in unimagined ways would still arise. If the 14th Amendment were intended in principle to be as capacious as the court has interpreted it to be over the last half-century, then the principle would apply in this case even if the specific subject matter (same-sex marriage) was something that the framers of the amendment had never considered. That’s exactly what courts are supposed to address: circumstances that do not perfectly fit expectations about whatever law is in question. This would still be an originalist argument, but it would be an originalist argument that said the principle that the framers of the 14th amendment established was more far-reaching than they could have envisioned, something hardly unexampled in the annals of law. No legislature “intends” specific outcomes in unimagined cases, but it can “intend” a general rule. The question is how general, and whether the same degree of generality in this area of law covers same-sex marriage as well as interracial marriage.
If law is the intention of the legislator, there’s obviously a problem whenever intended principles are radical and intended applications are limited.