@PedroTamaroff An outline of a nice way to prove that the complement of a saturated multiplicative set is a union of prime ideals is as follows:
(1) Given a multiplicative set $S$, define it's saturation $\hat S=\{x\in A: \exists y, xy\in S\}$. Show that $\hat S$ is saturated.
(2) Saturations relate to localization as follows; if $S^{-1}A=T^{-1}A$, then $T\subseteq\hat S$.
(3) The prime ideals of a localization $S^{-1}A$ are of the form $\mathfrak pS^{-1}A$ where $\mathfrak p$ is disjoint from $S$.