Some quotes from Aquinas on the subject of human sexuality may be of interest.
Because it was created by God, it is impossible for sex to be evil in itself.
“. . . carnal union is the end of certain bodily organs. But that which is the end of certain natural things cannot be evil in itself, because things that exist naturally are ordered to their end by divine providence . . . . Therefore, it is impossible for carnal union to be evil in itself.”
(Summa Contra Gentiles III, 126, 2) = SCG
Prior to the Fall human beings would procreate by sex.
“God made man and woman before sin (Gn. 1,2). But nothing is void in God’s works. Therefore, even if man had not sinned, there would have been such intercourse, to which the distinction of sex is ordained.
. . .
Some of the earlier doctors, considering the nature of concupiscence as regards generation in our present state, concluded that in the state of innocence generation would not have been effected in the same way. Thus Gregory of Nyssa says (De Hom. Opif. xvii) that in paradise the human race would have been multiplied by some other means, as the angels were multiplied without coition by the operation of the Divine Power. He adds that God made man male and female before sin, because He foreknew the mode of generation which would take place after sin, which He foresaw. But this is unreasonable. For what is natural to man was neither acquired nor forfeited by sin.”
(Summa Theologiae I, 98, 2) = ST [italics added]
The pleasure in sex would have been greater prior to the Fall.
“. . . In the state of innocence nothing of this kind would have happened that was not regulated by
reason, not because delight of sense was less, as some say (rather indeed would sensible delight have
been the greater in proportion to the greater purity of nature and the greater sensibility of the body), . . .”
(ST, I, 98, 2 ad 3)
Sex in marriage is holy.
“Now a human act is said to be good in two ways. In one way by goodness of virtue, and thus an act derives its goodness from those things which place it in the mean. This is what “faith” and “offspring” do in the marriage act, as stated above (2). In another way, by goodness of the “sacrament,” in which way an act is said to be not only good, but also holy, and the marriage act derives this goodness from the indissolubility of the union, in respect of which it signifies the union of Christ with the Church.”
(ST, Suppl. 49, 4)
The intensity of pleasure in sexual intercourse is not against the order of reason in human acts.
“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.” (ST, II-II 153, 2 ad 2)
The enjoyment of sex is increased by love.
“Now, such enjoyment [of sexual relations] is increased by the love of the persons thus united.”
(SCG, III, 125, 5)
Sexual intimacy increases love between husband and wife.
“Furthermore, the greater the friendship is the more solid and long-lasting it will be. Now, there seems to be the greatest friendship between husband and wife, for they are not only united in the act of fleshly union, which produces a certain gentle association . . ., but also in the partnership of the whole range of domestic activity.” (SCG, III, 123, 6)
Sexual sins are not the worst sins.
“Gregory [the Great] says (Moral. xxxiii, 12) that the sins of the flesh are less grievous than spiritual sins.” (ST, II-II, 154, 3)
The intensity of pleasure can diminish responsibility in sexual sins.
“The sensual pleasure that aggravates a sin is that which is in the inclination of the will. But the sensual pleasure that is in the sensitive appetite, lessens sin, because a sin is the less grievous according as it is committed under the impulse of a greater passion.” (ST, II-II, 154, 3 ad 1)