Although I have not personally seen this happen, impotence has been reported in the medical literature following myelogram with contrast material called Abodril or methiadol sodium, but is RARE. It's likely due to irritation of the spinal cord roots and subsequent inflammatory reaction called arachnoiditis. But another question is: could all the other complications of N/V severe HA, etc... be a psychologic cause of the impotence? Once everything resolves, it's quite possible that your erectile dysfunction will also improve.
As for the numbness in the "lateral 3 fingers" (this would mean the thumb, forefinger, and middle) it's in the right distribution for nerve damage from the C7 herniation. This means that the nerves coming from the C7 area of the spinal cord control or provide sensation to those fingers.
My advice to you would be to see a neurologist for both problems. A formal neuro exam would be very important to evaluate the degree of complications from the myelogram in your legs and point you in the right direction for management of the disc herniation. Best of luck.