The source for this info is here: Father John A. Hardon, S.J. Archives. I clipped this from the rather long discussion.
Outside of marriage and, for married people, outside of their relationship as husband and wife, venereal pleasure may not be directly sought or stimulated or enjoyed, and every deliberate excitation of such pleasure (if fully consented to) is a grave sin. Venereal pleasure is in a class by itself. It is felt when the sex organs are aroused and involves a stirring sensation of the organs of generation. Thus it differs from merely sense feelings, like inhaling the fragrance of a rose or enjoying a good meal; it also differs from sensual pleasure, as some call it, which refers to such experiences as a general “good feeling” or rise of emotion (not in the sex organs) that comes from almost any contact with someone who is loved.
Venereal pleasure is complete when it terminates in orgasm; it is incomplete in all other cases. Yet any kind of sexual indulgence, even incomplete, is gravely sinful if deliberately sought or experienced. This should be carefully distinguished from indirect venereal actions, which serve some other purpose than sex pleasure but may result in sexual excitation. Indirectly venereal actions are not sinful if a person has sufficient reason for starting or continuing them. Bodily needs of all kinds come under this category and may be summarily described as actions which it is reasonable to perform although sexual pleasure is expected or known to occur. This is an application of the principle of the twofold effect where arousal is permitted (but not indulged) for the sake of a proportionate good.
While there is no “minimum limit” in matters of sex, so that the amount of carnal pleasure a person derives does not change the essential gravity of guilt, yet the degree of awareness of what he is doing and the fullness of volitional consent he gives make a great difference. Unless there is complete awareness of mind and full consent of the will a mortal sin is not committed, no matter how strong the pleasure or how long it lasts or what reactions take place in the body.
It cannot be overemphasized that venereal pleasure in itself is not sinful, otherwise married people could not indulge it and, in fact, have a sacrament instituted by Christ to regulate its enjoyment. Even in the unmarried, such pleasure is natural and responds to a divinely-implanted instinct whose purpose is the noble one of leading men and women to conceive and procreate children. It may last for hours without a shred of guilt. But for the unmarried it is wrong to yield to that pleasure in the sense of wanting it in the body by knowing it is there, consenting to its presence and enjoying the genital stimulation which it gives. All three elements must be verified to constitute sin for the unmarried. It is indifferent whether the pleasure is deliberately procured or arises spontaneously; what is forbidden is the intentional yielding to an excitation of the generative organs.