Calico cats, with their orange, black and white coloration, are almost always female. The reason why has to do with genetics. The gene for black or orange coat color is on the X chromosome. Each patch of hair color starts with one cell. If that cell has an active orange X chromosome, a patch of orange will develop. If the cell has an active black X chromosome, black with develop. The black X chromosome and orange X chromosome are never active at the same time.
For more information, read the story Why Are Calico Cats Female?