There should never be such a march.
You are getting these replies because it is you who have missed the point.
The point of "Je suis Charlie" isn't that you can't be angered by sacrilege. It's that your anger doesn't justify murder. As an agnostic leaning more toward atheism, I am actually offended at some of the anti-Christian blasphemy on Charlie Hebdo's covers. Some of it is genuinely vile, whatever your affiliation. [And was deliberately printed to offend, which was the point of the authors.]
Piss Christ is also truly vile. I would not march in support of Serrano per se under any circumstances. But I would march in defense of his rights.
Some countries (aren’t Canada and the EUSSR among them?) are looking at adopting “blasphemy” laws to protect Islam’s sacred cows (including challenging the beliefs).