Look.  God create sex, and He created it in such a way as to involve pleasure.  God created food and associated pleasure with its consumption.  There's nothing wrong with pleasure ... only if the pleasure becomes an end in itself and is not ordered properly.  In fact, the greatest pleasure any human being will ever know will be in the vision of God.

OH FOR HEAVEN'S SAKE!

This thread perfectly proves that theology, and more succinct, moral theology is not for un-trained minds.

Causa finalis est; primus in intentione ultimus in executione.

Marital debt adherents' in lawful marriage must intend in all their acts, including those preparatory acts, and those terminating acts to INTEND to not frustrate the final cause of the marital debt itself. PERIOD Their is but one primary end. For our sake, God has permitted his infallible magisterium to enumerate for our thick heads and for the purpose of eradicating the pestilence of moral heresies, other ancillary final causes of marriage.
The funny thing about the final cause is that it is the first intended, last executed. There are a great deal 'acts' in between. Because, omne agens agit propter finem, the agents of this act if they are to utilize (use) the act (that is to say intend its end, they must permit the primary final cause; children. God does not fault any human being because part of man has deteriorated due to age or physical nature or from some other natural congenital evil which makes the natural and primary end of the marital act less likely. INTENT IS IN THE WILL. It is not in the body, though of course man being complex, has a corporal element.

Human beings are composites; body and soul, partially in actu and in potentia. If anyone reading this does not know these distinctions, then you ought not talk about any philosophical or spiritual subjects because you can't possibly understand.

Either sex which does not intend by an act of the will to be open to the magnificence of God's creative hand in the marital embrace, either by employee means via res artificialis, or res naturalis, does not intend the end. This is by definition abuse, (ab usus; to take away the end). The final cause is thus frustrated. This is always mortally sinful.

A man or a wife in licit marriage does not sin by engaging in the debt insofar as they in their wills and thus in the direction of their corporeal parts permit the spontaneous hand of the Divine Will to work.  If anyone says or believes something other than this, you need to talk to a priest because you are either incapable of understanding or unwilling--matters little the distinction.   The willful or obstinate misuse of reason causes scandal and it can block man's final cause--and this is an abuse as well.  

sɛҳuąƖ intercourse and lust come from the original sin; without the original sin human beings would have been multiplied without the use of sex.

 

Sounds like you're an Albigensian.

That lust came from Original Sin, concedo.  That pleasure came from Original Sin, nego.  Pleasure itself is GOOD.  Pleasure becomes lust when it is DISORDERED.

The old law is abrogated but if God said that a given action was a sin against natural law in the Old Testament, then it is a sin in both Testaments!

But the Church does NOT teach that it is against natural law.  That's just your invalid spin on the Old Testament.  In fact, Casti Conubii EXPLICITLY states that it is NOT "contrary to nature" to have intercourse with the secondary ends of marriage in mind even when the primary is not materially attainable.