Note that acetylene is neutral and while we can speak of the oxidation numbers of its atoms, we cannot speak of the molecule's oxidation state.
If we break up the #C-H# bonds we get #2xxH^+#, and #{C-=C}^(2-)# (carbon is more electronegative than hydrogen, so when you (for the purposes of assigning oxidation number) break this bond you put a formal #+1# charge on hydrogen, and a formal #-1# charge of carbon.
In fact, the acetylide unit #{C-=C}^(2-)# occurs as calcium carbide, #CaC_2#, which is an important industrial feedstock.
More reduced forms of carbon include ethylene, #H_2C=CH_2#, #C^(-II)#, and the methylene unit of a carbon , #-CH_2#, #C^(-II)#. Oxidation state assignments are of course formalisms; they do not have real significance other than what we assign for them. When we break a #C-C# bond in such a process, we conceive we get #2xxC*#, i.e. neutral carbon radicals.
