Do you ever call a cup a glass if it’s not actually made out of glass?
When you ask for a glass of water, do you expect the container you receive to actually be made of glass? Would you be offended if someone gave you a plastic cup instead of a glass one?
If it has a handle, is it no longer a cup or a glass? Would it then be a mug or a stein? What about a chalice or a grail? What do those look like?