also Japanese culture is different from Occidental ones. Like here we are used to kiss and sometimes even going further on a first date, some Japanese couples don't even share the same bed after marriage.
So for Japanese standards it might not be slow at all.
The weird thing is that it isn't like they don't wanna, some media does portrait some characters craving for that.
Like for us marriage might have a different meaning altogether than for them. Like our parties and receptions instead of something more quieter and reserved that seems to be the norm over there.