23 Nor, in that the Law orders a man to be purified even after intercourse with a wife, does it show it to be sin: unless it be that which is allowed by way of pardon, which also, being in excess, hinders prayers. But, as the Law sets many things in sacraments and shadows of things to come; a certain as it were material formless state of the seed, which having received form will hereafter produce the body of man, is set to signify a life formless, and untaught: from which formless state, forasmuch as it behooves that man be cleansed by form and teaching of learning; as a sign of this, that purification was ordered after the emission of seed.
For neither in sleep also does it take place through sin. And yet there also a purification was commanded. Or, if any think this also to be sin, thinking that it comes not to pass save from some lust of this kind, which without doubt is false; what? Are the ordinary menses also of women sins? And yet from these the same old Law commanded that they should be cleansed by expiation; for no other cause, save the material formless state itself, in that which, when conception has taken place, is added as it were to build up the body, and for this reason, when it flows without form, the Law would have signified by it a soul without form of discipline, flowing and loose in an unseemly manner.
And that this ought to receive form, it signifies, when it commands such flow of the body to be purified. Lastly, what? To die, is that also a sin? Or, to bury a dead person, is it not also a good work of humanity? And yet a purification was commanded even on occasion of this also; because also a dead body, life abandoning it, is not sin, but signifies the sin of a soul abandoned by righteousness.
Source: On the Good of Marriage (New Advent)