。
She knows absolutely nothing about sex.
Literal
She [topic-は] sex about [object-を] completely doesn't-know.
まるで paired with a negative verb (~ない) means 'not at all / entirely not' — full universal negation rather than partial. By itself まるで can also mean 'just like / as if' (まるで夢のようだ 'just like a dream'), but the negative pairing is its other major use; context picks the meaning. ~のこと ('about, regarding') softens the object — saying 'about X' rather than 'X' itself adds a slight remove that's natural with abstract topics. セックス is a mid-20th-century English loan; older or more euphemistic alternatives include 性 (せい) and 男女関係.