Objection 1. It seems that seminal loss does not hinder anyone from receiving the body of Christ: because no one is prevented from receiving the body of Christ except on account of sin. But seminal loss happens without sin: for Augustine says (Gen. ad lit. xii) that "the same image that comes into the mind of a speaker may present itself to the mind of the sleeper, so that the latter be unable to distinguish the image from the reality, and is moved carnally and with the result that usually follows such motions; and there is as little sin in this as there is in speaking and therefore thinking about such things." Consequently these motions do not prevent one from receiving this sacrament.
Objection 2. Further, Gregory says in a Letter to Augustine, Bishop of the English (Regist. xi): "Those who pay the debt of marriage not from lust, but from desire to have children, should be left to their own judgment, as to whether they should enter the church and receive the mystery of our Lord's body, after such intercourse: because they ought not to be forbidden from receiving it, since they have passed through the fire unscorched."
From this it is evident that seminal loss even of one awake, if it be without sin, is no hindrance to receiving the body of Christ. Consequently, much less is it in the case of one asleep.
Objection 3. Further, these movements of the flesh seem to bring with them only bodily uncleanness. But there are other bodily defilements which according to the Law forbade entrance into the holy places, yet which under the New Law do not prevent receiving this sacrament: as, for instance, in the case of a woman after child-birth, or in her periods, or suffering from issue of blood, as Gregory writes to Augustine, Bishop of the English (Regist. xi). Therefore it seems that neither do these movements of the flesh hinder a man from receiving this sacrament.
Objection 4. Further, venial sin is no hindrance to receiving the sacrament, nor is mortal sin after repentance. But even supposing that seminal loss arises from some foregoing sin, whether of intemperance, or of bad thoughts, for the most part such sin is venial; and if occasionally it be mortal, a man may repent of it by morning and confess it. Consequently, it seems that he ought not to be prevented from receiving this sacrament.
Objection 5. Further, a sin against the Fifth Commandment is greater than a sin against the Sixth. But if a man dream that he has broken the Fifth or Seventh or any other Commandment, he is not on that account debarred from receiving this sacrament. Therefore it seems that much less should he be debarred through defilement resulting from a dream against the Sixth Commandment.
On the contrary, It is written (Leviticus 15:16): "The man from whom the seed of copulation goeth out... shall be unclean until evening." But for the unclean there is no approaching to the sacraments. Therefore, it seems that owing to such defilement of the flesh a man is debarred from taking this which is the greatest of the sacraments.
I answer that, There are two things to be weighed regarding the aforesaid movements: one on account of which they necessarily prevent a man from receiving this sacrament; the other, on account of which they do so, not of necessity, but from a sense of propriety.
Mortal sin alone necessarily prevents anyone from partaking of this sacrament: and although these movements during sleep, considered in themselves, cannot be a mortal sin, nevertheless, owing to their cause, they have mortal sin connected with them; which cause, therefore, must be investigated. Sometimes they are due to an external spiritual cause, viz. the deception of the demons, who can stir up phantasms, as was stated in I, 111, 3, through the apparition of which, these movements occasionally follow. Sometimes they are due to an internal spiritual cause, such as previous thoughts. At other times they arise from some internal corporeal cause, as from abundance or weakness of nature, or even from surfeit of meat or drink. Now every one of these three causes can be without sin at all, or else with venial sin, or with mortal sin. If it be without sin, or with venial sin, it does not necessarily prevent the receiving of this sacrament, so as to make a man guilty of the body and blood of the Lord: but should it be with mortal sin, it prevents it of necessity.
For such illusions on the part of demons sometimes come from one's not striving to receive fervently; and this can be either a mortal or a venial sin. At other times it is due to malice alone on the part of the demons who wish to keep men from receiving this sacrament. So we read in the Conferences of the Fathers (Cassian, Collat. xxii) that when a certain one always suffered thus on those feast-days on which he had to receive Communion, his superiors, discovering that there was no fault on his part, ruled that he was not to refrain from communicating on that account, and the demoniacal illusion ceased.
In like fashion previous evil thoughts can sometimes be without any sin whatever, as when one has to think of such things on account of lecturing or debating; and if it be done without concupiscence and delectation, the thoughts will not be unclean but honest; and yet defilement can come of such thoughts, as is clear from the authority of Augustine (Objection 1). At other times such thoughts come of concupiscence and delectation, and should there be consent, it will be a mortal sin: otherwise it will be a venial sin.
In the same way too the corporeal cause can be without sin, as when it arises from bodily debility, and hence some individuals suffer seminal loss without sin even in their wakeful hours; or it can come from the abundance of nature: for, just as blood can flow without sin, so also can the semen which is superfluity of the blood, according to the Philosopher (De Gener. Animal. i). But occasionally it is with sin, as when it is due to excess of food or drink. And this also can be either venial or mortal sin; although more frequently the sin is mortal in the case of evil thoughts on account of the proneness to consent, rather than in the case of consumption of food and drink. Hence Gregory, writing to Augustine, Bishop of the English (Regist. xi), says that one ought to refrain from Communion when this arises from evil thoughts, but not when it arises from excess of food or drink, especially if necessity call for Communion. So, then, one must judge from its cause whether such bodily defilement of necessity hinders the receiving of this sacrament.
At the same time a sense of decency forbids Communion on two accounts. The first of these is always verified, viz. the bodily defilement, with which, out of reverence for the sacrament, it is unbecoming to approach the altar (and hence those who wish to touch any sacred object, wash their hands): except perchance such uncleanness be perpetual or of long standing, such as leprosy or issue of blood, or anything else of the kind. The other reason is the mental distraction which follows after the aforesaid movements, especially when they take place with unclean imaginings. Now this obstacle, which arises from a sense of decency, can be set aside owing to any necessity, as Gregory says (Regist. xi): "As when perchance either a festival day calls for it, or necessity compels one to exercise the ministry because there is no other priest at hand."
Reply to Objection 1. A person is hindered necessarily, only by mortal sin, from receiving this sacrament: but from a sense of decency one may be hindered through other causes, as stated above.
Reply to Objection 2. Conjugal intercourse, if it be without sin, (for instance, if it be done for the sake of begetting offspring, or of paying the marriage debt), does not prevent the receiving of this sacrament for any other reason than do those movements in question which happen without sin, as stated above; namely, on account of the defilement to the body and distraction to the mind. On this account Jerome expresses himself in the following terms in his commentary on Matthew (Epist. xxviii, among St. Jerome's works): "If the loaves of Proposition might not be eaten by them who had known their wives carnally, how much less may this bread which has come down from heaven be defiled and touched by them who shortly before have been in conjugal embraces? It is not that we condemn marriages, but that at the time when we are going to eat the flesh of the Lamb, we ought not to indulge in carnal acts." But since this is to be understood in the sense of decency, and not of necessity, Gregory says that such a person "is to be left to his own judgment." "But if," as Gregory says (Regist. xi), "it be not desire of begetting offspring, but lust that prevails," then such a one should be forbidden to approach this sacrament.
Reply to Objection 3. As Gregory says in his Letter quoted above to Augustine, Bishop of the English, in the Old Testament some persons were termed polluted figuratively, which the people of the New Law understand spiritually. Hence such bodily uncleannesses, if perpetual or of long standing, do not hinder the receiving of this saving sacrament, as they prevented approaching those figurative sacraments; but if they pass speedily, like the uncleanness of the aforesaid movements, then from a sense of fittingness they hinder the receiving of this sacrament during the day on which it happens. Hence it is written (Deuteronomy 23:10): "If there be among you any man, that is defiled in a dream by night, he shall go forth out of the camp; and he shall not return before he be washed with water in the evening."
Reply to Objection 4. Although the stain of guilt be taken away by contrition and confession nevertheless the bodily defilement is not taken away, nor the mental distraction which follows therefrom.
Reply to Objection 5. To dream of homicide brings no bodily uncleanness, nor such distraction of mind as fornication, on account of its intense delectation; still if the dream of homicide comes of a cause sinful in itself, especially if it be mortal sin, then owing to its cause it hinders the receiving of this sacrament.
Source: Summa Theologica (New Advent)