The demand for child sex is fueled by porn & fornication which in turn comes from decoupling sex from procreation, marriage, and family.
Ultimately, this has its source in idolatry and the rejection of God.
We're being shown in real-time the logical progression of liberalism. Rejection of Jesus -> Rejection of external divine authority and law -> acceptance of internal authority and do whatever I want -> acceptance of birth control -> sex is for fun -> acceptance of divorce -> acceptance of baby murder -> acceptance of child sex etc. etc. etc.
Forest and trees. Focusing to intently upon certain aspects of a larger whole can and most of the time will lead to ideas with incorrect conclusions. What your stating is not cold hard logic. You're starting with an assumption that it's only naturally pleasurable so that babies will be produced. This is your opinion. There are numerous reasons a man and women would desire each other's company in various forms. Sex and the pleasure that comes from it can certainly be selfish but that does not mean it always is. Great relationships, and I am only referring to a man and women in marriage, happen when there is unity and openness combined with the desire to serve each other and give of themselves to each other. You cannot isolate biology from the social and draw a proper conclusion. It doesn't necessarily end with passions morphing into degeneracy either. If men committed to faithfully loving their wives and wives the same for their husbands as I suggest I think just the opposite would happen. The real problem is not the reason why genitals exist, it's getting over the selfishness humanity is infested with. The pleasure is a unique gift one gives that bonds the givers. Sometimes it creates new life too. It's not to be passed off as simply necessary to get baby making going. I think we are seeing the consequences of sex being shunned as only for baby making and it's created failled social commitments and it's destroyed the oneness of married life resulting in mass misunderstanding of the wholeness and joy of knowing someone else in this world physically and socially so well that you consider yourself one with them.
It's a lot of mental gymnastics for "i want to have my jollies without the natural consquences." It's really simple.
The proper end of sex is procreation.
The proper end of eating is nutrition.
Both have pleasure associated with it, but it is not the proper end. The purpose of pleasure is so that you have sex, so that you eat, because procreation and nutrition are so important to life. If sex and eating were painful, then few would bother to do it.
If you say the proper end of sex is pleasure, then what follows is hedonism, fornication, rape, pederasty and other evils.
If you say the proper end of food is pleasure, then you get obesity and disease.