Wow, that's some concession. And proponents of gay marriage are willing to concede that heterosexuals have every right to marry someone of the same sex. Is that of any consequence to you? If not, the point is moot. Just as it's moot (not to mention transparently disingenuous) to say that homosexuals may marry someone of the opposite sex.
They aren't willing to concede to anything that doesn't conform to their idolatry of perversion.
Marriage certificate = birth certificate.
Secular, biological, no esoteric hobgoblins, no perverted ceremonious fakery - - crams the fallacious "separation of church and state" argument right back down their throats... (pun intended)