Does the ending mean that she can only...

Wait does the drug leave only 1/6 emotions, or was that a random number you made up?
k, to answer the question: yes, morning wood would work, just make sure he doesn't go to the bathroom for #1 first. For other times, I'm guessing she can search for a natural root that would serve similar function to viagra. Until then qu**ning might have to do.

And while all that's happening, she can be searching for additional natural drugs that can get him hyped up. Cocaine. Amphetammines. Caffeine. She needs to keep studying history how all that stuff is made and break down the cure. The cure is version 1.0 so probably not that perfected. I'm sure they can crack it, but until then
she'll have to "face" the problem with creative solutions...

Erections don't have to come from emotions alone, without emotions they'd still rear their head (so to speak) during rigorous activity or out of plain old necessity. Release has to happen one way or another, hence nocturnal emissions.