Objection 1. It would seem that no venereal act can be without sin. For nothing but sin would seem to hinder virtue. Now every venereal act is a great hindrance to virtue. For Augustine says (Soliloq. i, 10): "I consider that nothing so casts down the manly mind from its height as the fondling of a woman, and those bodily contacts." Therefore, seemingly, no venereal act is without sin.
Objection 2. Further, any excess that makes one forsake the good of reason is sinful, because virtue is corrupted by "excess" and "deficiency" as stated in Ethic. ii, 2. Now in every venereal act there is excess of pleasure, since it so absorbs the mind, that "it is incompatible with the act of understanding," as the Philosopher observes (Ethic. vii, 11); and as Jerome [Origen, Hom. vi in Num.; Cf. Jerome, Ep. cxxiii ad Ageruch.] states, rendered the hearts of the prophets, for the moment, insensible to the spirit of prophecy. Therefore no venereal act can be without sin.
Objection 3. Further, the cause is more powerful than its effect. Now original sin is transmitted to children by concupiscence, without which no venereal act is possible, as Augustine declares (De Nup. et Concup. i, 24). Therefore no venereal act can be without sin.
On the contrary, Augustine says (De Bono Conjug. xxv): "This is a sufficient answer to heretics, if only they will understand that no sin is committed in that which is against neither nature, nor morals, nor a commandment": and he refers to the act of sexual intercourse between the patriarchs of old and their several wives. Therefore not every venereal act is a sin.
I answer that, A sin, in human acts, is that which is against the order of reason. Now the order of reason consists in its ordering everything to its end in a fitting manner. Wherefore it is no sin if one, by the dictate of reason, makes use of certain things in a fitting manner and order for the end to which they are adapted, provided this end be something truly good. Now just as the preservation of the bodily nature of one individual is a true good, so, too, is the preservation of the nature of the human species a very great good. And just as the use of food is directed to the preservation of life in the individual, so is the use of venereal acts directed to the preservation of the whole human race. Hence Augustine says (De Bono Conjug. xvi): "What food is to a man's well being, such is sexual intercourse to the welfare of the whole human race." Wherefore just as the use of food can be without sin, if it be taken in due manner and order, as required for the welfare of the body, so also the use of venereal acts can be without sin, provided they be performed in due manner and order, in keeping with the end of human procreation.
Reply to Objection 1. A thing may be a hindrance to virtue in two ways. First, as regards the ordinary degree of virtue, and as to this nothing but sin is an obstacle to virtue. Secondly, as regards the perfect degree of virtue, and as to this virtue may be hindered by that which is not a sin, but a lesser good. On this way sexual intercourse casts down the mind not from virtue, but from the height, i.e. the perfection of virtue. Hence Augustine says (De Bono Conjug. viii): "Just as that was good which Martha did when busy about serving holy men, yet better still that which Mary did in hearing the word of God: so, too, we praise the good of Susanna's conjugal chastity, yet we prefer the good of the widow Anna, and much more that of the Virgin Mary."
Reply to Objection 2. As stated above (152, 2, ad 2; I-II, 64, 2), the mean of virtue depends not on quantity but on conformity with right reason: and consequently the exceeding pleasure attaching to a venereal act directed according to reason, is not opposed to the mean of virtue. Moreover, virtue is not concerned with the amount of pleasure experienced by the external sense, as this depends on the disposition of the body; what matters is how much the interior appetite is affected by that pleasure. Nor does it follow that the act in question is contrary to virtue, from the fact that the free act of reason in considering spiritual things is incompatible with the aforesaid pleasure. For it is not contrary to virtue, if the act of reason be sometimes interrupted for something that is done in accordance with reason, else it would be against virtue for a person to set himself to sleep. That venereal concupiscence and pleasure are not subject to the command and moderation of reason, is due to the punishment of the first sin, inasmuch as the reason, for rebelling against God, deserved that its body should rebel against it, as Augustine says (De Civ. Dei xiii, 13).
Reply to Objection 3. As Augustine says (De Civ. Dei xiii, 13), "the child, shackled with original sin, is born of fleshly concupiscence (which is not imputed as sin to the regenerate) as of a daughter of sin." Hence it does not follow that the act in question is a sin, but that it contains something penal resulting from the first sin.
Source: Summa Theologica (New Advent)