15-year-olds often make poor decisions. No doubt. Still, they need to be held responsible for the consequences of their actions. But often not to the same degree as adults.
Teachers are adults, so they are held to a higher standard, and teachers to a higher standard still. This is intuitive. A 15-year-old's ability to make decisions for which he should be held accountable doesn't make him any less worthy of protection from predatory behavior from adults, and doesn't make the adult's choice to exploit her position/influence/access any less reprehensible.
I am honestly baffled how anyone can think that holding teenagers accountable for criminal behavior somehow runs counter to criminalizing adults having sex with minors.