In reality he is not giving much by way of argument. It is the traditional 'ethic' of the seminary...the lesser of two evils....there is no sense that he is weakening the Roman Church's teaching about contraception. Indeed it's noted that he has chosen an example in which conception is not going to happen. Therefore the condom is not being used to prevent conception, but rather to prevent disease.
I say it is the traditional ethic of the seminary, because most of us who live in the real world see that this is a sort of slippery slope.
Where is the suggestion that it would be OK for the rapist to wear a condom...because it would be the lesser of two evils for the victim to not have to become pregnant as well as be raped.
Where is the suggestion that it would be better for the victim of poverty to not have a sixth, seventh or eighth child...particularly (when as is so often the case) the mother might well die leaving many children.
Most people, I suspect, won't be convinced that His Holiness is really living in the real world with this pronouncement.