Actually, porn gets people even more into the mood and you can get some new ideas (positions, different foreplay, whatever) from watching it. It isn't just about OH I AM SINGLE AND A FAT LOSER SO I WANT TO SIT BACK AND WATCH PORN, it's also about a couple watching it together and helping the sex even more because, face it, we're human and we are all going to get off on people screwing so it's going to get us horny. People who can watch people have sex on TV and not get hot off of thinking of themself and their man/woman doing it are just robots or have no sex drive whatsoever.
It does not replace sex. A guy or girl who is single can just masterbate without having the need to watch porn so it isn't as if single people really need it to get off on and when in a relationship, if you are horny, you don't need to sit back and jack off to porn, you can just screw your man/woman.