Not true, or at least not the whole truth. Marriage has always meant the union of two human beings. Marriage between black people and white people was prohibited in the old south because black people were not considered to be fully human. Interracial marriage was not forbidden. A white man could marry a woman who was Asian, or Latin American, or Native American.
Slaves were allowed to “marry” each other, but this was a corruption of the language, a minor sin to go along with the major sin of slavery. A “marriage” between two individuals who could not legally withhold their consent if their master ordered them to do so, or for that matter if their master decided to split them up, was not a real marriage, even if the law recognized it as such.
The kernel of truth in your assertion is that the statement by a white man that “I’m married” did indeed used to convey the information that he was most likely married to a white woman, and that the ability of the word “marriage” to convey that he conformed to the prevailing standards of racial bigotry has now been lost. But then, the prevailing standard of racial bigotry has itself been lost, and so we see that the meaning of the word changed along with a change in actual conditions.
Now, having read that you might be thinking: If it’s ok for the definition to change when people become less bigoted against black people, why is it not also ok when they become less bigoted against gay people? Especially given what you said:
Back when my addiction was raging I argued against this point at length. If people start posting to this thread again, I’ll dig up the link, even though that will put the monkey back on my back for sure. But to put it succinctly, gender does matter to the overwhelming majority of us, when it comes to finding someone to have an intimate relationship with, in a way that race does not. Take yourself, for example. As a gay man, you want to have an intimate relationship only with another man. No woman will do, no matter how many fine qualities she has. Does that mean you are bigoted against women? Of course not.
Now, consider: A white man might rule out ever marrying a black woman because he is bigoted against black people. If it happens that he grows as a person and becomes less bigoted, he might then become open to the idea of marrying a black woman. Now, a straight man who is bigoted against gay people certainly doesn’t want to marry another man. And if he should also happen to grow as a person and cease to be a bigot, he still won’t want to marry another man, because that’s just what being a heterosexual means.