Really? You... you're going to force me to... (swallows) spell it out? Sigh...Oh, I must know now...
As I mentioned before, the Sahc-diy-deuh's main purpose is that of procreation, as the Tamearin's level of fertility jumps. And this effects their mood. And because they are both feeling an access of emotions and are extra fertile... ahem, well... let's just say that Astasha isn't the first ship captain or Commander to ban physical relations in her ship, so this rule was made to give Tamearin woman an outlet for certain... needs. Needs that, if unfullfilled, would likely cause them to snap, violently or... (cough) sexually...