This subject does go on a bit but the contribution on the 1754 seems clear
and does not say marriages and children were retrospectively illegal. The
wording suggests to me that after that date outside the blessings of the
church were not valid. The church would normally be Anglican and ultimately
other marriages would have to be registered officially. Is not the simplest
interpretation that Christening is the naming ceremony and baptism the
acceptance of a person into the body of a church? The two are often
combined nowadays in the Anglican sacrament.