Things I notice while studying the Korean language
Me, too, but, surprisingly the Chinese chose Number 2. The first character means "flirting with or teasing a woman."
姦, like nearly every other Chinese word that has seen any use, has several meanings and nuances, including: 간사하다, 간음하다(yours), 강간하다, 속이다, 훔치다, 어지럽히다, and 어지럽다.To me, it doesn't seem like much of a stretch to go 속이다 → 훔치다 → 강간하다 → 간통하다.The 뇨 is an interesting dictionary relic, but, as far as I can tell, it's never been used. Just cited in old dictionaries like the 玉篇 and 龍龕手鍼.
And, I suppose I shouldn't say this, but it doesn't seem like much of a leap from a group of women to "속이다," either. ^^