From Webster's Revised Unabridged Dictionary, © 1996, 1998 MICRA, Inc.:
miscegenation
\Mis`ce*ge*na"tion\, n. [L. miscere to mix + the root of genus race.] A mixing of races; amalgamation, as by intermarriage of black and white.
Marriage has always been defined as one man, one woman. Perverting the meaning now is no better than perverting the meaning then.
But your analogy fails on many bases, not just the above.
Homosexuals, as a group, have never been enslaved, lynched, equated to 3/5 of a human being or denied rights based on the color of their skin.
Homosexuals can do what they want, they can even marry as long as the marriage fits the definition of marriage. What they should not be allowed to do is use judicial oligarchs to make law and elected executives to interpret law.
My guess would be a person who practices miscegenation. You know, like a physicist would practice physication . . .
Its a two-for-one deal; newly created law & freshly minted words. Hot off the Mayor's desk, timely and fresh as the new day. All that puttering with legislation is just so inconvenient. Why waste time on this silly concept of Rule of Law?