I imagine it's not so much about how exposed it is in the moment (because evolutionarily we didn't have to worry about things like guns), but rather how quickly you can move to protect it when you're starting off vulnerable and someone has suddenly moved to attack you.
Protecting your genitalia, when standing/sitting, is as simple as bending forward slightly and pinching your thighs together. Protecting your neck requires, at the very least, rotating your whole upper body and leaning to interpose your shoulders between you and your attacker. Much harder to do in a third of a second when you see a kick flying at you.
Protecting your genitalia, when standing/sitting, is as simple as bending forward slightly and pinching your thighs together. Protecting your neck requires, at the very least, rotating your whole upper body and leaning to interpose your shoulders between you and your attacker. Much harder to do in a third of a second when you see a kick flying at you.