In the introduction, give a simple elaboration on the concept of public morality.
In the body, write about aspects such as what is accepted within the realms of public morality, whether it is static and indestructible by quoting few examples. Influence of public morality on legislations and the changes with time such as legalisation of same sex marriage etc. can be mentioned.
Conclude by stating that society evolves with time and thus the public morality also takes its shape, however certain historically sustainable values must be preserved as well.