Is it permissible for a married couple to watch porn films, as one of them suffers sexual tepidity and does not become sexually aroused except by watching such things?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

You should know it is invalid in the Shareea'h that one seeks to achieve a lawful objective through doing what is prohibited. When the end is permissible, the means must be permissible as well. What is mentioned in the question is an impermissible action, as it implies looking at what is prohibited by Allah. Add to this, when a person gets accustomed to watching such promiscuous scenes, he will gradually lack religious prudence. The Prophet, sallallaahu ‘alayhi wa sallam, said, "Do not look at the thigh of anyone, living or dead."

It should be noted that sexual excitement is the consequence of foreplay with one's wife including caress, kissing, etc. You should also know that sexual frigidity has its causes. So you have to firstly avoid looking at what is prohibited by Allah and visit a physician in order to diagnose your case and give you the needed treatment.

Allah knows best.