Can too much exercise lead to decreases in sexual stamina?

Fatigue. Too much exercise can lead to fatigue. Good daily exercise combined with good nutrition should lead to increased sexual stamina.
Yes. If you exhaust yourself, you will have less power and less endurance while your body is trying to recover from the overtraining... If you exercise correctly--slow transition and avoid overtraining, you should get to a point that you feel better, have more power, and have healthy sexual drive as well as stamina.