Writing in the Mail on Sunday, Cardinal Keith O'Brien said that although the Church's view on the issue had been "ridiculed or ignored", there was increasing evidence to justify its concerns.
The Church has been outspoken in its criticism of the plans, warning that the institution of marriage will be eroded and that churches could face lawsuits if they refuse to conduct same-sex marriage ceremonies.
He pointed to reports from Brazil of a civil partnership between three people that was officially recognised in the state of Sao Paolo.
Although the Scottish Government has promised that churches will not be forced to host gay marriage ceremonies, Cardinal O'Brien dismissed the pledge as an "empty phrase".
He pointed to the example of Denmark, where the Danish Parliament voted earlier in the summer to force churches in the established Evangelical Lutheran Church to perform same-sex marriage ceremonies inside their sanctuaries. The law came into force in June, overturning the previous decision to allow churches to opt out.
"Along with others, we have asked what can stop further erosion and destruction of the meaning of marriage once it ceases to be the relationship between a man and a woman and becomes the recognition of a commitment made by adults who love one another," the Cardinal wrote.
"Such redefinition must surely, logically allow for multiple partners to enter into 'marriage' we have warned."
He continued: "Along with others, we have warned that opt outs from legislation can easily be overturned. If Parliament votes to protect religious celebrants from being compelled to conduct same sex marriages it can just as easily vote to overturn that protection."
The Cardinal further warned that the promise to exempt churches from performing same-sex marriages would not stretch to the use of their premises for same-sex celebrations or receptions.
He noted the decision in April by the city council of Hutchinson, Kansas ,in the US, which brought into force a new rule stating that buildings available for public use would not be allowed to discriminate on the grounds of sexual orientation or identity.
The city stated at the time: "If a church has a parish hall that they rent out to the general public, they could not discriminate against a gay couple who want to rent the building for a party."
The Cardinal concluded: "Each of these cases confirms and expands on the fears and concerns we have expressed. They show where this debate is going and our media and our politicians have almost universally ignored them.
"While I pray that our elected leaders will sustain rather than subvert marriage, I can assure the Scottish Government that together with Scotland's silent majority, we will continue to do everything we can to convince them that redefining marriage would be wrong for society."