If that were anywhere near true then real life women would never get laid. Especially average and below average looking women. Trust me, men prefer the real thing. If a man ever loses his attraction to you and is watching porn then chances are he would've lost his attraction to you either way.
That said, if it is making you really insecure then you're not doing either yourself or your bf any favors by bottling your angst away inside and hoping it just goes away. You perceive a problem and it needs to be communicated. Calmly, explain to him what you explained to us, be completely honest about it. It is making you very insecure because of what happened in your last relationship, and you can't bear the thought of the same thing happening again.
That said, don't expect him or make him feel obligated to just kick the porn to the curb. This isn't a guilt tripping session, nor is it a bitch-fest. It's an open communication about your feelings, he needs to at least know what they are. There are solutions. For example, maybe you can join in on him watching some porn? Maybe the two of you can have sex with some hot, steamy porn in the background? Maybe that will make you more comfortable with it and show you that you have nothing to worry about? If you're keeping porn out of your sex life out of insecurity then you are missing out.
Or maybe just talking to him about it and getting it off your chest will make you feel better. Maybe you can find some reassurance that your last relationship will not repeat itself.