Thinking of getting pregnant while having untreatable cancer. Want to pass on a piece of myself. Am I wrong?

Natural thought. It's natural to have these feelings. However, remember that a child will only have your genes, not your parental involvement. The cancer may also make for a difficult pregnancy, since cancer is just as big a resource drain as a fetus. Perhaps there is another way to ensure your legacy? Discussion with a therapist, clergy, or other friendly advisor can help.
Procreative Instinct. It is not wrong to satisfy your procreative instinct. The ethical questions pertain to handing down to the offspring a genetic predisposition to cancer if present in your situation and having a parent to raise the child when you are not around. The safety of and potential defects the fetus may have as you go through treatment during the pregnancy may also be of concern.
Baby without mother. Put yourself in the shoes of your baby who would not have the benefit of having a loving mother.