Contraception encourages people to engage in sex when without it they would not. Obviously this is not the case for everyone, as unplanned pregnancy has always happened, but what the easy access to contraception does is encourage fornication where previously most would have avoided it (because of the repercussions, whether that be a child or an abortion).
Beyond that, a sex life that has makes no allowance for children IS sinful itself, because it is denying the very basis of where sex comes from and is about; specifically procreation. There are all manner of other things tagged on to sex, but that is its origin and core.
Finally, in separating sex from procreation we separate it from puberty....and that opens up children to being sexualised (which we can clearly see in society today). Thus when contraception is the norm, paedophilia becomes percieved to be less deviant.