I'm not actually sure that it's actually the sex that kills the player as much as... She literally somehow destroys the world while doing it, if you look at what kind of events go on etc. Plus she flat out says 'while your world ends'.
She saying that isn't the world ending, it's just the matter of fact, for whatever reason, your body destroys itself because of... pent up sexual energy? And that is cured by the.. non-MALO sexual interaction..? Seems like a weird story choice.
