It’s because they think they are going to die from the hangover. It’s the urge to procreate that happens when people are faced with potential death.
Hmm. I know that nitric oxide can cause erections:
The role of nitric oxide in erectile dysfunction: implications for medical therapy - PubMed
and I really thought I’d read that nitric oxide was a byproduct of alcohol breakdown. But a quick search doesn’t turn up that information.
Am I just wrong, or am I thinking of something else?