lust after, act indecently, lewd
媟 centers on improper desire and indecent behavior: lusting after someone, acting lewdly, or engaging in morally impure acts.
媟 combines 女 (woman) with 枼, which likely contributes the sound. The exact historical development of 枼 in this character is uncertain, but the phono-semantic structure is typical.
A woman 女 beside a complex shape 枼 that looks like a tangled, messy affair. Picture a woman entangled in a lewd, indecent situation.
For セツ, imagine a set of indecent acts: set -> セツ, and the woman is caught in that set of lewd behaviors.