And hear all along I thought they were nitric oxide modulators...
No, they don't affect nitric oxide levels. The release of nitric oxide is partially involved in having an erection. It activates the enzyme guanylate cyclase which forms cyclic GMP. It is this compound that leads to relaxation of the blood vessels that supply the corpus cavernosum. These drugs inhibit an enzyme that breaks down cyclic GMP, thus increasing its level and allowing the erection to proceed. If you want to directly increase nitric oxide levels, there are other strategies you can follow.