If you want to make me angry, don't insult me, just show me two things I believe that seem to contradict each other and watch me try to figure out either how they fit, or which one isn't true.
This has been the case with 1 Corinthians 7:12, in which Paul says something along the lines of "I, not the Lord, say this". Obviously, I believe the entire Scripture is the word of God (2 Timothy 3:16-17). Thus, if Paul says something in the Scripture, the Lord says it. And yet, here Paul says "I, not the Lord, say..." So, is the Lord saying the Lord did not say, then says it anyway? Is this the one time in Scripture where God allowed His Apostle to just make stuff up? Or is this a case of Paul making an error, and so God inspired him to say "I, not the Lord", so that we would know to take his words with a pinch of salt? This apparent dilemma lead to many sleepless nights for a very long time.
That is until one day, Sam Shamoun and David Wood received this very question on a live stream they did on YouTube, and God bless Sam Shamoun, he actually answered it! It turns out, when you think of this in terms of Jesus' earthly ministry as opposed to that which He speaks through His inspired vessels, it all makes sense.
As we know, there are many things Jesus is recorded to have said while on earth, but also many things we do not have Jesus' spoken opinion on. There is such a thing as a "red letter Christian", basically meaning if Jesus did not speak on a specific topic, Jesus didn't care (and some even take His silence as proof of their view). But obviously, the red letters both confirm the black letters, and cease to make sense without them. So, one could say Paul, not the Lord, said homosexuality is an unnatural lust (Romans 1:26-27). Jesus didn't directly mention homosexuality, but Paul did. So, was Paul going off the script in Romans 1? No, he was just preaching that which Jesus did not touch during His earthly ministry.
Similarly, Jesus did preach that spouses must not leave each other, and that they must remain unmarried if they do leave (v10-11, cf. Matthew 19:1-10; Mark 10:1-12). But, He did not address what to do in the case of an unbelieving spouse. Paul was not preaching that which Jesus had addressed during His life, but was rather expounding upon it in his capacity as an inspired Apostle.
Thus, the apparent contradiction is resolved. Paul is not saying "this is not the word of God", but "this is not what Jesus said during His life".