Numbness to the penis is most often caused by a reduction in blood flow to the pubic region. The main culprit of this symtom is your saddle, specifically, the nose of the saddle putting pressure on the veins and nerves of the perineum. When you sit on a narrow bicycle saddle, there is a 66 percent reduction in blood flow (Goldstein, 1997).
Because you're leaned over on the rollers and are not standing, adjusting, etc., you are putting more consistent pressure on that area than you're used to when riding on the road. I would recommend that you lower the nose of your saddle, so that you're sitting more on your butt bones, rather than your perineum. Also, a more anatomical saddle might be a wise investment.