I think the refractory period is a hormone response and not tissue damage/fatigue. It could be ‘healed’ in a similar way horniness, PMS, pregnancy, tiredness (but not the need for sleep), hunger, and moods can be ‘healed’.

It could also be a peripheral autonomic feedback response, in which case it could be ‘healed’ in a similar way to suppressing automatic breathing, bowel movements, automatic reflexes, and possibly the fine tuning of balance.

Such a spell could porobably exist, alongside a suite of other weird spells that were likely developed first, like Calm Emotions or unnamed but almost guaranteed to exist contraceptive spells. Some debuff spells of a unique variety could be quite interesting too, with breathing suppression being the rare effect that would require an intelligence save.

A potion would probably be the cheaper option though; plain old erection juice should suffice.