Addiction to pornography is a serious danger sign in someone you are contemplating marrying. You would be wrong to ask him to do many things that he may enjoy but you are NOT wrong in asking him to reduce or eliminate this "hobby" or addiction. He may need psychological counselling to assist him.
One thing can lead to another until it totally backfires on you. Obsessing on naked pictures of other women is downright disrespectful to you and you shouldn't tolerate it. Some intermittent browsing of such material may be OK but there has to be a limit. Most men enjoy looking at an occasional naked lady other than their s/o but they don't make it an everyday thing.
Talk to him one more time and if he won't get help, postpone the engagement or just cancel it. It is far better for you to go through the hurt and pain now than for you to set yourself up for even greater hurt later. If you marry him full well knowing he has this problem, you will have absolutely no basis for complaining later...and I promise it will only get worse after you're married. Right now, he's supposed to be at his best behavior.
If you want to read about a similar experience of another person who posted on this site yesterday along with some of the responses, go here:
http://www.loveshack.org/forums/show...threadid=17355
Just remember, without serious intervention this problem won't get better and it very definitely has the potential for destroying the relationship...and doing serious, hopefully temporary, damage to your life.
Good luck.