Tell them Poser is just a tool. In the right hands it can produce art. CG art.  Yes, it is CG art. It's done with a computer... it involves graphics... it's CG art. Simple. All software packages are simply tools.

Most of the people who have a problem with Poser have a problem with the pre-made models. Either they have a problem with the human form (some people have been brainwashed and conned) and/or they don't like the easily recognizable looks of un-morphed Poser and DAZ models.

If the proper composition, color and lighting rules have been followed and it evokes a feeling, who can't call it art?

CG Talk/CG Society is frequented by more modelers than anyone else, so there is a strong anti-pre-made anything bias going there. Very few there have done anything remotely close to the quality of Poser or DAZ models and doubtfully ever will (if it was fast and easy to model, we could all do it - it's not the same as learning how to draw and paint, which also require talent). And if someone does, the idiots there will claim it's Vicky or Michael unless they've already been anointed as modeling gods. I've seen it happen a few times.

My experience has been the folks with the problems have 1) never made any art, 2) are nut jobs or 3) are wannabes parroting the prevailing wisdom of the forums. Real world pros do not have that bias or lost it a long time ago.

Art is subjective and many from other art fields may have problems with it. You also have the people who prefer real humans over CG humans in movies. Lots of biases out there. So be prepared to have a thick skin about your CG art.