There will be different groups of arguments for decriminalization:
Type A: The "criminal" behaviour has to be
accepted by the society (f.e. homosexuality).
Type B: It may not be accepted, but it is not
the purpose of government to control it (f.e. unsafe sex between
adults and AIDS).
Type C: It may not be accepted, and the
government has a right to intervent, but criminal law, especially in
it's current form, is not the best way to intervent.
There are different problems which have to be considered to detect if
a law is useful: