In context, this is speaking of someone else's wife. Or do you think it sinful to look at your own wife with lustful intent?
You are right to say that porn wasn't directly prohibited, of course it's been an "invented evil"(Romans 1:30).But without question it is evil, sinful and destructive.
Porn is a lust/desire/heart issue, as well as a love issue.
For the married. As you later quoted "if we burn with passion" (1 Corinthians 7) Paul says marry. Become one in flesh in love, and with most hope one in Spirit in Christ, God's will. In this way both can be devoted to the Lord together. As Paul also said this a difficult road/endeavor as honest married Christians will attest to. Obviously it's an offense to desire others sexually for those who are married, as Jesus said it's in effect adultery. And that betrayal is felt by parties involved.
There is the danger for the married to desire thier spouse more than God. An idol between the heart of us and God. Any good thing that God created we can turn into an idol. The good thing itself is not bad just how it sits in our heart. Do we worship the creator or things He created? (Romans 1:25)
For those that are single if you burn with passion, marry; but don't set up idols and get addicted to porn, or even wanting a spouse so badly it consumes you. The Devil is trying to tempt us into being consumed by any and everything but God. It all comes down to this.
"And he answered, "You shall love your Lord your God with all your heart and with all your soul and with all your strength and all your mind, and your neighbor as yourself." Luke 10:27
Christ values us so much, more than we can even imagine, pouring His blood out. Turning what He loves into,an object of obsession and gratification... Does any parent want that for their child? Of course not! There is nothing but lies, idols and desecration and destruction in porn. To demise of many... God help us!! Have mercy!