Why is it such a big deal?

Because Miley is supposedly a role model for young girls, and the idea of a young teenager enjoying her sexuality (as is natural) doesn't jibe with parent's efforts to "shield" their kids from any interest in sexuality (also natural).

That, and sex sells, and there are obviously a lot of men who are interested in Miley Cyrus. People like to be able to be self-righteous and seem morally indignant while simultaneously enjoying the titillation of forbidden sex.