There was no carnal knowledge in this wedlock, because in sinful flesh this could not be without carnal desire which came of sin, and which He would be without, who was to be without sin; and that hence He might teach us that all flesh which is born of sexual union is sinful flesh, seeing that Flesh alone was without sin, which was not so born.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: de Nupt. et Concup., i, 12