What are some things other than STDs that can cause itching in the penis? (sorry if this is inappropriate)

Pre Cancer. Fungal infection, dermatitis, allergic responses to clothing, allergies in general are most common. However, do not forget that venereal warts and precancerous skin conditions can also occur. Intraepithelial neoplasia, squamous cell cancer, paget's disease. Get it checked out.
Yeast. Actually a fungal infection is more common to cause itching. Usually near the foreskin and glands. Try an antifungal cream on the area and try to keep the area dry. If no improvement see a physician.