That's called disagreeing. Their arguments are:

* They were legally transferred by the govt in power in Greece at the time

* Elgin rescued the marbles from destruction from an apathetic state

* Greek government of today can't speak for the state of Greece then

If you don't consider these valid, then yes, you stand on the "return" side.

To me, the discussion about the means of acquisition is only relevant if the debate is "Should the UK be legally compelled to return the marbles", but if the debate is "Should the UK return the marbles" then I don't feel its relevant either way.