Traditional Catholic moral theology holds that such types of activities can be venial sins if sought excessively for themselves without being ordered towards the primary and secondary ends of marriage, but only become mortal sins when the activities are not ordered at least by way of virtual intention towards the primary ends of marriage. In marriage, when there is no attempt to subvert the ends of marriage, i.e. the couple is not trying to block conception, that virtual intention is almost always there. With that said, excessive indulgence in venereal pleasures for their own sake can lead to patterns and habits of venial sin (and we know that even venial sin should never be taken lightly) which then can in turn open up the soul for falls into mortal sin. Ideally, such activities should be expressions of love and affection between a married couple, who in turn are married and love one another with the intent of procreating children. Even if they cannot procreate, however, due to infertility or advanced age or whatnot, these can be legitimately ordered towards the secondary ends of marriage. But when they are sought directly in and of themselves, and are not properly subordinated to the primary and seconds ends of marriage, they can be venially sinful. If, however, they are sought in such a way that they are inherently anti-procreative, they can become morally sinful.
Some of these Puritan / Jansenist types think that pleasure is inherently sinful. Yet why is it that actions which when performed outside of marriage are sinful become licit when done within marriage? Because the sin is in the WILL; all sin is ultimately in the will. If the will is aligned towards the laws of God with regard to the ends of matrimony, there is in many cases no sin whatsoever. Sin isn't because of the pleasure itself; it's because the pleasure is sought in a way that's contrary to the laws of God. People become obsessed with the "pleasure" part of sins when they should be focused on the "will" part of the sin. Satan was sent to hell immediately precisely because his sin was so deeply rooted in the will (being an angel), but God puts up with us human beings sinning over and over again because human beings are so weighed down in our fallen natures that these sins are not as radically rooted in our wills as they were for Satan.
People also become excessively focused on sins against the flesh, or sins of pleasure, when in many cases these sins come from weakness, but then do not focus on sins like lack of charity against one's neighbor, sins of pride, and other things which very often displease God much more than excessive indulgence in pleasure because these sins are much more deeply rooted in the will and involve malice. God has more compassion on sins of weakness than he does on sins against charity or sins of malice. Just see how Our Lord treated a poor fallen sinner like St. Mary Magdalene vs. how He treated the pride of the pharisees. In fact, I am of the opinion that God sometimes allows people to fall into sins of the flesh in order to instill humility in them, so much more does pride and selfishness and lack of charity displease God than do sins of weakness.