Not actually true, so far as I can tell.
It’s all here, with links to references in case you are one of the MassResistance haters.
http://www.massresistance.org/romney/index.html#SSM
Sorry, but it is true, Romney did implement what the legislature would not. Romney instituted same sex marriage in Massachusetts. The courts told the legislature to do it but they were too afraid touch it. Even though no one (neither the courts nor the legislature) told Romney he had to do so, he ordered the clerks of the court to issue marriage licenses.
Since he’s the one who did it, Romney’s the one who is responsible.
Now did he actually officiate at a gay wedding? That I don’t know anything about.
But did he choose two gays raising a child to be celebrated as parents of the year? Yes, he did.