b) You somehow aren’t “mature” enough or experienced enough to have an erection (which is completely irrational). You fear she will laugh at you or call you a silly little boy for not having an erection (this is actually a defense for her b/c she is offended and doesn’t want to show weakness) and thus you will be humiliated and [in your mind] set apart from others. This makes you vulnerable.
As you can see, every single example leads to fear of vulnerability, which leads to fear of death.

Literal interpretation:
An erection enables sexual intercourse to take place. It is a male’s instinct and need to spread his sperm/genes to as many females as possible in order to reproduce (after all, we are animals). To not have an erection when with a woman you would otherwise have sex with is to make sex impossible (for the time being). Thus, the animal in us fears it. If you cannot reproduce you are an outcast among others who can, which makes you vulnerable. Anyone having de ja vu?

Conclusion: fear of death.