Well, this kind of situation is generally approached in a very broad way. After all, for the practical result it does not make much difference if the driver is distracted by intimate stimulation or by reading a book, and the legislators do not usually want to make a full list of all possible distractions1.
So, for example, the Victoria Road Safety Act 1986 states (page 96):
17A Obligations of road users
(1) A person who drives a motor vehicle on a
highway must drive in a safe manner having
regard to all the relevant factors.
[..]
(2A) For the purposes of subsections (1) and (2) and
without limiting their generality, the relevant
factors include the following—
[..]
(g) the physical and mental condition of the
driver or road user.
(3) A road user must—
(a) take reasonable care to avoid any conduct
that may endanger the safety or welfare of
other road users;
(b) take reasonable care to avoid any conduct
that may damage road infrastructure and
non-road infrastructure on the road reserve;
(c) take reasonable care to avoid conduct that
may harm the environment of the road
reserve.
Apart from that, and depending of the situation, there is the possibility of being subject to anti-indecency laws if the act can be observed from outsiders (e.g. you are riding in a motorcicle).
1If you have a very specific list, then other activities that are equally dangerous but that are not in the list are not illegal.
That does not prevent that sometimes the laws mention certain practices separatedly because:
They want some specific behavior to be punished differently (e.g. driving while drunk).
They want to make clear that an specific behavior is illegal (e.g. using the phone while driving is illegal even if you claim that it does not impair your driving).
