Yin is feminine in textbook definition. Chaos "gave birth" to two children, a feminine characteristic, while Pandora did not. This is a joke explanation, don't take this too seriously. Also, feminine and masculine here does not necessarily mean in a biologically gendered way, it could also describe traits of a person. In this sense, feminine entails traits such as "the Moon, moody, quiet, introverted, caring, nurturing, etc." while masculinity entails "the Sun, extroverted, communicative, active, intelligence". A man can have feminine traits, and a woman can have masculine traits. Chaos can be seen as "feminine" because he's quiet, introverted, and openly sadistic. A little note how females can be seen as the worst kind of monsters or aliens to men in classic portrayals. While Pandora possesses extroverted, energetic, intelligent, and active personality. A fatal mistake is seeing them as only the yin and yang, while they both possessed their own yin and yang. Chaos, despite being quiet and scary, is dominant, calculating, prideful, charismatic and competitive - traits of masculinity. While Pandora (especially when he's around Chaos), is more quiet, obedient, and passive - traits of feminity.
I was strolling around Tumblr for some crumbs, wondering if anything good shows up, I don't know if you're still around, but I had fun reading your recap and analysis!