To a virgin, for Christ could be born from virginity alone, seeing He could not have an equal in His birth. It was necessary for our Head by this mighty miracle to be born according to the flesh of a virgin, that He might signify that his members were to be born in the spirit of a virgin Church.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: de san. Virg. cap. vi.