Objection 1. It would seem that virginity is unlawful. For whatever is contrary to a precept of the natural law is unlawful. Now just as the words of Genesis 2:16, "Of every tree" that is in "paradise, thou shalt eat," indicate a precept of the natural law, in reference to the preservation of the individual, so also the words of Genesis 1:28, "Increase and multiply, and fill the earth," express a precept of the natural law, in reference to the preservation of the species. Therefore just as it would be a sin to abstain from all food, as this would be to act counter to the good of the individual, so too it is a sin to abstain altogether from the act of procreation, for this is to act against the good of the species.
Objection 2. Further, whatever declines from the mean of virtue is apparently sinful. Now virginity declines from the mean of virtue, since it abstains from all venereal pleasures: for the Philosopher says (Ethic. ii, 2), that "he who revels in every pleasure, and abstains from not even one, is intemperate: but he who refrains from all is loutish and insensible." Therefore virginity is something sinful.
Objection 3. Further, punishment is not due save for a vice. Now in olden times those were punished who led a celibate life, as Valerius Maximus asserts [Dict. Fact. Mem. ii, 9. Hence according to Augustine (De Vera Relig. iii) Plato "is said to have sacrificed to nature, in order that he might atone for his perpetual continency as though it were a sin." Therefore virginity is a sin.
On the contrary, No sin is a matter of direct counsel. But virginity is a matter of direct counsel: for it is written (1 Corinthians 7:25): "Concerning virgins I have no commandment of the Lord: but I give counsel." Therefore virginity is not an unlawful thing.
I answer that, In human acts, those are sinful which are against right reason. Now right reason requires that things directed to an end should be used in a measure proportionate to that end. Again, man's good is threefold as stated in Ethic. i, 8; one consisting in external things, for instance riches; another, consisting in bodily goods; the third, consisting in the goods of the soul among which the goods of the contemplative life take precedence of the goods of the active life, as the Philosopher shows (Ethic. x, 7), and as our Lord declared (Luke 10:42), "Mary hath chosen the better part." Of these goods those that are external are directed to those which belong to the body, and those which belong to the body are directed to those which belong to the soul; and furthermore those which belong to the active life are directed to those which belong to the life of contemplation. Accordingly, right reason dictates that one use external goods in a measure proportionate to the body, and in like manner as regards the rest. Wherefore if a man refrain from possessing certain things (which otherwise it were good for him to possess), for the sake of his body's good, or of the contemplation of truth, this is not sinful, but in accord with right reason. On like manner if a man abstain from bodily pleasures, in order more freely to give himself to the contemplation of truth, this is in accordance with the rectitude of reason. Now holy virginity refrains from all venereal pleasure in order more freely to have leisure for Divine contemplation: for the Apostle says (1 Corinthians 7:34): "The unmarried woman and the virgin thinketh on the things of the Lord: that she may be holy in both body and in spirit. But she that is married thinketh on the things of the world, how she may please her husband." Therefore it follows that virginity instead of being sinful is worthy of praise.
Reply to Objection 1. A precept implies a duty, as stated above (Question 122, Article 1). Now there are two kinds of duty. There is the duty that has to be fulfilled by one person; and a duty of this kind cannot be set aside without sin. The other duty has to be fulfilled by the multitude, and the fulfilment of this kind of duty is not binding on each one of the multitude. For the multitude has many obligations which cannot be discharged by the individual; but are fulfilled by one person doing this, and another doing that. Accordingly the precept of natural law which binds man to eat must needs be fulfilled by each individual, otherwise the individual cannot be sustained. On the other hand, the precept of procreation regards the whole multitude of men, which needs not only to multiply in body, but also to advance spiritually. Wherefore sufficient provision is made for the human multitude, if some betake themselves to carnal procreation, while others abstaining from this betake themselves to the contemplation of Divine things, for the beauty and welfare of the whole human race. Thus too in an army, some take sentry duty, others are standard-bearers, and others fight with the sword: yet all these things are necessary for the multitude, although they cannot be done by one person.
Reply to Objection 2. The person who, beside the dictate of right reason, abstains from all pleasures through aversion, as it were, for pleasure as such, is insensible as a country lout. But a virgin does not refrain from every pleasure, but only from that which is venereal: and abstains therefrom according to right reason, as stated above. Now the mean of virtue is fixed with reference, not to quantity but to right reason, as stated in Ethic. ii, 6: wherefore it is said of the magnanimous (Ethic. iv, 3) that "in point of quantity he goes to the extreme, but in point of becomingness he follows the mean."
Reply to Objection 3. Laws are framed according to what occurs more frequently. Now it seldom happened in olden times that anyone refrained from all venereal pleasure through love of the contemplation of truth: as Plato alone is related to have done. Hence it was not through thinking this a sin, that he offered sacrifice, but "because he yielded to the false opinion of his fellow countrymen," as Augustine remarks (De Vera Relig. iii).
Source: Summa Theologica (New Advent)