No access to contraceptives means that people who do not intend to have children, but engage in sex, will have children. These children will be unintended or unwanted and may not be cared for either through disinterest or lack of ability to provide for them.
A child who grows up in that sort of circumstance is not kept safe, may succumb to disease, may spread disease, will not be educated, and will have difficulty providing for themselves.
A common rebuttal is that people should just not engage in sex if they are not intending to have children. This is maybe ideal in some moral frameworks, but whether it's ideal or not, it's not the reality. The reality is that some people will engage in sex anyways, and some people will be forced to engage in sex against their will. While it might be a goal to prevent these things from occurring, the reality is that they occur, and methods to mitigate the negative impacts of their occurrences are better than just finger-wagging.
That said, if your argument is more along the lines of "The Pope says that it is wrong, and the Pope espouses God's will." then I can't really refute it. I can just disagree.
In any case, if people can choose whether they want to have a child or not, then contraception just helps them make that choice. If they shouldn't engage in sex for non-childbearing reasons, then they can struggle with that decision as well. However, when succumbing to that results in an unwanted childbirth, it does more than just punish the parent, it now involves the child and the whole community.