The seventh house is opposite the ascendant, the place that sets on the western horizon as the first house rises. Everything the medieval catalogue puts there has the same shape: it is what the native faces. Lilly gives it marriage, the wife or husband, love questions, contracts and bargains, lawsuits, and open enemies.
Putting the spouse and the declared enemy in one house is not a joke at marriage's expense. The seventh is the house of the other party, whoever that party is and whatever the dealing between them. A marriage, a partnership, a contract and a suit at law are all arrangements in which a second person is named, present and able to answer. One house carries all of them, and a reading tells them apart by the planets involved and by the question asked, not by the house.
Venus is the general significator of the spouse in the medieval and Renaissance tradition, as the planet of attraction, agreement and the terms on which people are drawn together. The particular significator is the lord of the seventh, the planet ruling the sign on that cusp, and it is the particular significator that carries detail: where the spouse is found, in what condition, and whether the two significators come to an aspect and by what motion.
That last point is the working heart of a horary question on marriage. The astrologer looks for an applying aspect between the lord of the first and the lord of the seventh, and reads the manner of the meeting from the manner of the aspect.