In fact, Romney simply declared gay "marriages" to be licit in Mass., although the Mass Supremes' ruling specified that such license would require a new State law.
Jan. 2, 2006 Boston Globe reports Romney issued special Governor’s ceremonial marriage licenses to 189 same-sex couples in 2005 (including to homosexual activist state senator), claiming he did not refuse because he was evenly applying the “statute”. [Note: There is no new statute establishing same-sex marriage.]
Admittedly, the situation there is a bit confusing. But the fact is that Romney was under no obligation to implement gay "marriage": he did so voluntarily, even though he claimed that the Court's ruling compelled him to do so.