Beauty is good. Sex is good. Sexual pleasure per se is good. However, sleeping around is not. Lust is not.

Why is beauty good? Because beauty is convertible with the good. What is beautiful is good and vice versa. In this case, a physical beauty is good as it is nearer physical perfection. There are different kinds of beauty, but this is the one currently under discussion.

Why is sex good? Because it the actualization of human nature as procreative being. No one has a right to sex, of course, but the dimensions of procreation not directly involving intercourse may be realized otherwise, though also not something you can impose (procreation isn't just sex that successfully realizes its intrinsic end of new human life, but the raising of the children afterward, and we can be mother and father figures in purely spiritual ways, through adoption, or in relations with younger generations; it's why Catholic priests are called fathers, and the heads of convents mothers, for example). The unitive end (which includes bonding) of sex, which can only be understood through the lens of the complementary procreative potentials and the procreative raison d'etre of the sexes and their manifestation in human nature, is also a good of sex, and hence pleasurable.

Why is sexual pleasure good per se? Because as noted above, sex is good per se. Good things are pleasurable. Of course, this does means we can suffer from deficiencies, excesses, and disorders in this regard. We can become insensate and inert to goodness in a way that we derive little or no proportionate pleasure from the good. Depressed people are an example. We can experience excess pleasure disproportionate with the good in question. Having an orgasm eating a slice of pizza is an obvious example. We can also experience disordered pleasures, taking pleasure in acts that are depraved and evil. Rape or non-procreative sex or sadism and masochism are examples. But given the objective good of sex, it is normal for it to be pleasurable.

Why is sleeping around evil? Because it is opposed to the intrinsic "structure" and end of sex. It is intrinsically exploitative, reducing people to objects to be used for pleasure, and this destroys one's ability to respect human life and love human beings and relate to members of the opposite sex. It destroys the person doing it. Note that you don't need to sleep around to achieve the same effect. People who indulge in pornography are actually worse. At least the person sleeping around is sleeping around with people. The porn user lives in a fantasy land, locked within himself, reducing people to fleeting phantasms of sexual gratification, never relating in anyway to a human person. As disordered as sleeping around is, porn users are worse. Abstain if you can't marry. You'll live.

Why is lust evil? Lust is essentially sexual desire divorced from and opposed to reason, which is to say, human nature as rationally knowable. Human nature determines our objective good. It is our duty to recognize it and subjectivize it so that it reflects and is aligned with objective reality. Indulging a subjective that is at odds with the objective is evil. Appropriate strong sexual attraction or desire toward a spouse is not what I have in mind, only disproportionate or disordered sexual desire. One of the effects of lust is that it blinds the mind. The object of sexual desire is reduced to an object of selfish sexual gratification, and nothing besides that object is perceived. It dulls the wits. When we speak of chastity, we mean sexual restraint and purity of intention. It is a matter of discipline and self-control. It is only through chastity that sexuality can be expressed in a healthy and morally licit manner.

Love is the willing of the good of someone. Thus, to love yourself is to will your own objective good. To love another is to will their objective good. Since we are social animals, it is bad for us to harm or exploit or will evil on another person. Doing evil destroys the evil doer.

Our age is an age obsessed with sex, and yet also an age that is full of sexual disorder and impotence and pornography and loneliness and selfishness. This is not an accident. Sexual immorality destroys a society, making a man a slave of his passions and incidentally someone who is easy to control.