As far as this vaccine, there is already a version ready to go for males. But if you gave it to males (who spread the disease) there would then be no reason to vaccinate females. That's why this particular vaccine is so insidiously evil. They could spare the girls this trauma...

Surely this would also be equally valid:
there is a version ready to go for females. But if you give it to females (who are in danger from the disease), there would be no need to vaccinate males. That's why this vaccine is so insidiously evil. They could spare the boys this trauma...

It's not complicated. You protect the people directly at risk. Besides, a teenage boy would claim the sky is green if he though it would help him get laid. You really think they wouldn't lie about a vaccination?