Does sugar cause sexual weakness in men?

Yes and no.

The No.

Sugar is needed to maintain a healthy blood pressure. If your sugar is too low, then your blood pressure will be too low. Since the male penis relies on blood pressure to form an erection, low blood sugar is a problem here.

The Yes.

Too much sugar can cause reduced production of testosterone and therefore reduced sexual drive. It can also cause Diabetes, which can further cause loss of drive.

The advice is to eat a healthy, balanced diet. Just eat as much sugar as you need (ie natural sugars from fruits) to maintain a healthy blood pressure and avoid too much sugary junk food to prevent loss of drive.