Gay marriage is too recent a phenomenon to be addressed by any Council.  As for trusting the Church, if their arguments depend on papal infallibility and not the facts on the ground, they are not to be trusted on natural law matters.  Doctrine on the actual faith is a different matter.  It evolved when Rome was an Aryan backwater that did not attend the great councils of the fourth century and it was made up of democratically elected bishops who voted the same way.