Thanks, Oddity-- but the Thema Mundi and planetary joys really seem to me like separate systems. It's gotta be more than just blood. As you know, the 5th is the house of one's children, as well; so there's got to be more to it than simple 1:1 correspondences. Venus joys in the 5th.
The Thema Mundi has been called a teaching tool, presumably on domiciles. I looked it up in Brennan's Hellenistic Astrology, and rightly or wrongly the later Hellenists attributed it to Imhotep (Asclepius) and Hermes via Nechepso and Petosiris. This seems like the Hermetic tradition. Firmicus Maternus (book 3) has quite an exposition on it, relating the planets' feminine and masculine domiciles and positions to their aspects to the sun and moon.
FM gives an ordering of the planets related to historical periods of humanity, from a more primitive state to an apparent pinnacle in his day.
There are several different explanations for the origins of the joys, some more rational and some more mystical. Brennan (p. 342) thought that the joys probably pre-dated the Hermes texts. He notes parallels between the meanings or significations of the houses and the joys.
My own theory is that the joys derived from ancient Egyptian religion and its passage of the sun through different stations at night, and the soul in the afterlife. For example, Manilius gave the 12th house to the Roman god Typhon (cf. our word typhoon) who seems like a cognate god for the Egyptian Seth, who threatened the new sun with chaos. Seth was the "great malefic" in the Egyptian pantheon, so he could reasonably linked to Saturn.
Venus, joying in the 5th house, was cognate with the Egyptian goddess Hathor, who ruled childbirth and pleasure.
So much is unknown about the origins of horoscopic astrology, in part because some of the material had the status of religious secrets, revealed only to initiates.