Ah yes, some refer to this as "culturally Catholic". My mum would take us to church but never receive communion, for example.

What is interesting here is the decline of Christianity over time, given that we often hear we are a country founded on Christian principles and despite considering ourselves as having a separate church and state, the former continues to influence the policy and legislation of the latter.

I think what is really happening is that cultural norms have largely been based on Christianity in this country, but most of the "good" norms can exist without it with the worse ones falling by the wayside (in fact, this is what secularisation means to me). This is probably a reason for the rise of "no religion", because people have access to information and can communicate with others and come to their own conclusions about universal morality, so to speak. Since most of these moral values align with religions to some extent, but since religions also demand we ascribe to other values that are inconsistent or poorly articulated (e.g., homosexuality as a sin, marriage as God's institution), people may reject those values and choose to not follow a religion.

If you think about it like this, it's no wonder a lot of the baby boomers and older generations fear cultural change. To many of them culture is - perhaps unknowingly - tethered to religious ideas, rather than continuously shaped by new knowledge and ideas. If you're living in a country that is 3/4 Christian, the culture of that country will reflect that.

Recently, my dad (who is a cultural Catholic but could be considered more of an agnostic in later years) explained to me that when he was a young adult, marriage was considered a gesture of your commitment to another person, and getting engaged to a woman was expected for those in committed relationships as a gesture towards that woman's family (as opposed to living "in sin", which might imply that the woman was not worth marrying). Marriage was also an important sacrament under Catholicism as it led to the creation of a family (think of how many baby boomers use the term "bastard", or can tell you a story about a shotgun wedding or child born out of wedlock). There is a lot of overlap between religion and culture, here.

To many of these people like my dad, people's decisions to not marry, whether they stay single or are in otherwise-committed relationships, doesn't make sense - but it is almost as if they can't put their finger on why. It's because the cultural ideas around marriage have shifted, and the religious ones have not. I suspect some people have rejected religion for not keeping up with those shifts.

A great example of this is the Margaret Court Incident. Any baby boomer you talk to will likely defend her right to express her opinion and relate this to Australia being founded on Christian principles (in some form) and lament the change in Australian society. The idea that a foreign-born, openly gay man can be the CEO of a major Australian company (that was also the first commercial airline in the world, mind you) publicly advocates for marriage equality must blow their minds.