Exactly. It's a trick to make creeping legislation easier
If anyone wanted, they could have simply changed the definition of 'minor' without first changing this bill. The change that was now passed has absolutely no effect on a 'possible' (lol) change of the definition of 'minor'.
Because a threshold for the age limit could easily have been added. The fact that it wasn't is deliberate.
Of course it was. It is much more efficient for both reading and maintaining laws and bills when common terms are defined in one place, instead of in every single bill (or sentence) where the term is used.
you retreat to convenient example of a 17 + 18 year old couple which no one that I'm aware of has any real concern over
How would you propose to write a bill that mirrors the situation of a 17+18 year old couple that 'nobody has any real concern over', and still is strict enough for your liking?