definition of "unnecessary sex"

Anything that is more than what YOU do...

But yes, the Victorians did think that way....officially. Behind the scenes they were a naughty lot.
   The attitude was exemplified in a book of advice....(which I have an original copy of)

Plain Facts for Old and Young:
Embracing the Natural History and Hygiene of Organic Life,
by J. H. Kellogg, M. D. (1892, Burlington, Iowa, U.S.A.)

This was old 'tie their hands' Kellog....who urged parents to spy on their kids to prevent them from engaging in 'the solitary vice', and if necessary, tie their hands to the bed post at night.

(you can read the whole thing online, it says.)