Have you asked him if he's ever had anything up his ass other than his doctor's digit in a prostate exam? If the, uh, shoe were on the other foot, would he be comfortable or willing to have his butt invaded and penetrated, repeatedly?... He probably HAS watched too much porn on this, and is hot to do it. It's your body, and if you are serious about this guy and your future with him, you should be honest in your refusal of something you don't want.
I cast my vote with the firm ""I don't want to" approach...but, there also is much stength in just saying "NO!" and not yielding to his "but it you really loved me, you'd do this for me..." argument...if you haven't heard that one yet, you probably will...
He should have enough respect for you to accept your refusal for anal sex, and if he doesn't...you should lose him for good. Any guy that forces you to engage in something that you strongly don't want is not a keeper, and not worthy of you, either. If he argues that you don't know if you like it or not until you try it, you can say that you tried it with a vibrator once out of curiousity, and it just hurt 'way too much, and you are totally not interested in going there again.. You can refuse this without confessing or admitting that you have allowed a guy to do it previously. That's your personal business and something you should not feel required to share...If this guy continues to push for anal sex in spite of all your protests and refusals...you should give serious consideration to ending the relationship...he obviously doesn't respect you and he probably never will, and if you give in, he's likely to respect you even less, and think that he can get anything and everything he wants...and you just don't want to even think about going there...believe me.
Good luck with your dilemma...I hope things work out for you...