"Why not, Why not,..." joke

A thief breaks into a house occupied by three women,
namely the grandmother, the mother and the daughter.
The thief wastes no time in tying up the three women
and ransacks the entire house. Upon leaving he sees
the daughter in a particular position, which stimulates
him and in an instance rapes her.
Spurred on by his new passion he goes on to rape the
mother. Meanwhile the daughter senses that who his
next target might be and asks the thief' are you
going to do this to my grandmother as well? '
'Why not, why not,. ...' comes the reply in a faint
feeble feminine voice