mate, copulate, good
姤 centers on pairing or joining, especially in a marital or sexual sense: mating or copulating. The meaning 'good' likely derives from the idea of a favorable match or union.
姤 combines 女 (woman) with 后 (ruler/queen), likely suggesting a woman joining with a ruler or a noble match. The exact historical development is uncertain.
A woman 女 and a ruler 后 come together in a formal union: mating or a good match.
For コウ, imagine a couple (a good match) going together: 'co-' as in cooperate, cueing コウ.
gou (hexagram 44 of the I Ching: coming to meet)