Well, it kind of is. Let's look at your argument again:
You are saying that the parallel bnetween interracial marriage and same-sex marriage doesn't work because at no time could blacks and white NOT get married. They just couldn't marry between races. Whereas, gay men and women can't get married, thus the analogy breaks down.
But, of course, gay men and women can get married. They just can't marry someone of the same sex.
In other words, the parallel condition is met because while no race was ever prevented from marrying at all, neither has a person of any sexual orientation ever prevented from marrying at all. In both cases, people have always been able to marry people within the legally acceptable parameters, just not necessarily of the people of their choice, weither of a different race or of the same sexual orientation.
Ergo, this also defeats your economic argument because the pool of people capable of entering marriage will not change any more than it did when interracial marriage was legalized.