In over 30 years of church attendance, I've never once heard it mentioned during a sermon, and have attended about 20 different Prostestant churches in different states. (Only a guest speaker said it was the number #1 problem)
It sounds like your guest speaker was expressing his own opinion; or maybe he was referring to the growth of the pornography industry over the past couple of decades. It's a multi-billion-dollar-per-year industry and that means A LOT of people are buying the stuff. And it stands to reason that if a lot of people are buying it, then at least some of them must be churchgoers.  But think about it: These simple facts, recited in a sermon, might get your senior pastor into a lot of trouble with his congregation; could even cost him his job.  \n\nNow are you really surprised that he's never mentioned it?
1. Christians receive spiritual gifts when they are saved, one of those gifts is prophecy. So in answer to this question many Christians have the gift of prophecy. \n\n2. The Old Testament matters very much. It is part of the whole council of God. The entire bible is relevant and harmonious from Genesis to Revelation.\n\n3. Who is picking and choosing? If you are saved and consider one portion of the bible more important or relevant than another, you are missing out. \n\n4 My Suggestion? Read and study the bible for yourself. Then ask questions and check the answers against the Word of God.