Ptolemy's chapter on marriage, in the fourth book of the Tetrabiblos, does not begin from the seventh house. It begins from the luminaries, and it begins asymmetrically: in a man's chart the Moon is examined, and in a woman's chart the Sun.
The logic is the sect logic that runs through the whole work. The Moon is the night light and stands for the wife a man will have; the Sun is the day light and stands for the husband a woman will have. From there Ptolemy reads the timing and the character of the union from the sign the relevant light occupies, from the planets configured to it, and above all from Venus, Mars and Saturn, which between them supply desire, friction and delay.
The distance between that and Lilly's seventh house is a fair measure of how much changed between the second century and the seventeenth. Hellenistic astrology has twelve places, but it does not hang the significations on them the way the later catalogues do. The lots, the sect, the condition of the planets and the aspects carry more of the weight. The house catalogue is a medieval consolidation, transmitted through Arabic astrology into Latin, and by Lilly's day it had become the first thing a student learned.
Both approaches are practised now. The traditional revival of the 1990s put the Hellenistic and Ptolemaic methods back into working use beside the house method, so a modern reader is likely to meet both, and should not assume that a disagreement between two books is an error in one of them.
Sources TetrabiblosClaudius Ptolemy with A History of Horoscopic AstrologyJames Herschel Holden