Concupiscence

Sexuality can truly be understood only if  the dignity of the human person understood. A human person should be understood in the light of his divine nature and the infinite capacities he is endowed with - to love, express love and mature in love. In the absence of the this, sex can only be perceived as a commodity or a capacity to pleasure oneself or on the other extreme, as an ugly, detestable thing, leading to evil. Below examples substantiate the two extremes.

1. If I encroach into my neighbor’s garden, pluck and eat the apples growing there because it gratifies my desire and my hunger, I certainly have unlawfully utilized what belong to the other. But on the other hand, if I become a part of the family [marriage] that owns the garden, I become the owner of the garden and I commit myself to do what is necessary to care for the garden and the apple trees until I breath last. To share in its fruit is the gift of that self giving. By doing that not only am I edified but also helps the tree to regenerate by freeing seed from the apple to find its purpose in the good soil. 
2. If someone in an attempt to demean birds, gathers bird droppings and highlights its unpleasant nature to me; in order to make me believe that birds are bad, I will most likely to believe if I have not known birds yet.  

Sex is not the limited act of intercourse as we have understood it to be. Instead it is the "unique nature" of a person that distinguishes between man and women. The very awkwardness we feel looking at the word “sex” on a form, next to the field where we write our gender, is the result of our ignorance of mystery of  sexual union as God created it to be.  Union of a man and women is so much more than intercourse. It is the union of two mutually complementing natures. Intercourse is only a part of this bigger union.  Intercourse achieves its purpose and meaning only in the context of marriage and procreation.

The selfish act of utilizing only the pleasure component within the sexual union to gratify one’s concupiscence, avoiding the intended responsibilities is surely the abuse of the of sexual freedom. The intended responsibilities are responsible parenthood and mutual kenosis. [Akin to the self emptying love of Christ to his church]

One may ask, if sexual urge is my deepest desire to love the other, why not I love all? Since sexual union is a gift primarily to love and procreate, it is ought to be within bounds of a covenant [inseparable marital commitment] in order to preserve and protect the well being of the spouse and the offspring. Only if the parents unite in an mutual offering of unconditional love, can they freely and responsibly accept children- the resultant gift of sexual union. [You may refer Pope John Paul's teaching, the "Theology of the body" and expound on the four essential characteristics of [sexual] love namely - free, total, faithful and fruitful.]

While discerning the morality of the sexual deeds, the question to be asked is “what is the motivation?”. If the answer is “responsible parenthood” you are most likely to be on the right path. But if the answer is “pleasure" evading the responsibilities intended, then make no mistake, it’s a sin.

Sin is defined as an abuse of the freedom that God gives to created persons so that they are capable of loving Him and loving one another [CCC 387].

Concupiscence is defined as an ardent, usually sensual, longing or lust. The concept is most commonly encountered in Christian theology, as the selfish human desire for an object, person, or experience. For Christians, concupiscence refers to what they understand as the orientation, inclination or innate tendency of human beings to long for fleshly appetites, often associated with a desire to do things which are proscribed. [Wikipedia]