Hirugao is the more serious, somber show about affairs, and Fukigen na Kajitsu has more romanticized affairs with a lot of intimate scenes. Both have lonely wives with disinterested husbands, as well as awkward mother-in-law relationships where the mother-in-law is too involved and doting. Both involve wives contemplating/weighing the value of an affair before it starts, but Hirugao focuses more on those affected and morals.

While the main theme of Secret is not infidelity, both have darker romances of people falling for people they did not intend to fall for, and intense hate-to-love elements.

Both have infidelity themes, mature elements, and passionate attraction, while doing a good job of telling all sides of the story.

Both are stories of people who end up loving the wrong people, with main themes of infidelity, but tells all sides of the story.

Both are adapted from popular shoujo mangas, with a confident, seemingly cold lead that pursues a shy female lead. Both have physical contact elements.

Both have darker, more mature themes and romances involving temptation and irresistible attraction with intense chemistry.

Very similar themes of anxiety about marriage and getting older, and both feature a surly ML who loves to lecture women on how to live their dating lives.

Similar themes of older woman/younger man, and in both the FL is struggling to get over a past/prior love.

Both of these have similar themes of a woman in her 30s presented with three men, struggles involving dating a younger man, and anxiety over marriage and getting older.

Both of these series have similar themes of a lovechild seeking revenge as an adult for tragic childhood cruelties by their father's families. Both have hidden identities and love interests that threaten to derail their revenge plans. The stepmom and stepson personalities are very similar. Hua Jai Sila is more romance-oriented, and Money Flower is more revenge-oriented with a higher production value.

These both have similar themes of water gods. My Poseidon reminds me a lot of The Bride of Habaek.

Both can be fun, light watches if you're not looking for something too serious.