It's not just the Priests. California, Florida and other states in the US have held plebiscites to ban gay marriage. In Florida the majority in favour of the ban was over 60%. Opposition to gay marriage is grounded in primeval instincts that recognise marriage primarily as a vehicle for the birthing and rearing of children and Gay men can't have children. Romantic marriage is a relatively new phenomenon.
It is an important milestone on the road to equality, but not equality.
The first principle to establish is recognition, and this bill achieves that. Full same-sex marriage is evitable and will come, probably within 5 years. Polls clearly show that same-sex marriage is backed by over 60% of the electorate, and the vast majority of people under the age of 40%. The principle opposing demographic is those over the age of 60, and those people, by definition, are a declining segment as a larger proportion of that demographic die daily than in other demographics. Once the the support hits 70% than the margin is enough to deliver a win. (Usually on social issues, there is a swing to the conservative side or circa 15%. So 62% is not yet a winnable margin. But once it hits 70% it is reasonably guaranteed, and on polling trajectories that 70% may well be hit within 4-5 years.)
"Irish citizens . . . on ratification of the Treaty could be forced to become Euro soldiers." Sinn Féin claim on Maastricht in 'Democracy or Dependency' p.6. in 1992.