why does being fatigued affect the strength of your erection?
An erection is not caused by muscles but blood filling the erectile tissue.
So it’s not affected by muscle fatigue in the classical sense.
But when you are fatigued your body likely doesn’t go into sexy time mode causing an erection, it wants to sleep or rest and not do another exhausting acticity
It’s a mix of psychological and physical factors. Basically it’s harder for your brain to get/stay in the mood when tired. That also means that even if your mind was actually in sexy time modeyour body, your body’s survival automation systems would reroute blood flow and energy towards other functions better suited to your current low-energy state