Fiancée watching Tranny porn after promising he wouldn't...

Here's the deal: I am engaged, the big date is August 30th, 2008. About 10 months from now. I love my fiance with all my heart and I know he loves me....but I have some major fears and insecurities about one issue in particular. My fiance watches trans sexual pornigraphy.
It started way back when we were dating. I had only known him for about two months at the time and he asked me to house-sit while he was away on business. While I was at his house, I was on his computer and couldn't find a web page that I had vsited earlier that day. I went into the history to find it (I am NOT computer savvy at all, but my brother showed me this one trick)> Anyway, I did find my page, but I also came across A LOT of trans sexual porn sights.....ie: men dressed like women with their "parts" intact. Now, I consider myself to be fairly liberal and open sexually. I am actually ok with porn in small doses and enjoy it myself from time to time, but this particular content confused me. I confronted him about it and he said he was just curious and it would NEVER happen again. I felt sooooo much better.
Skip to three months ago: We are now engaged. This time I was into the hard drive on our computer and I was trying to find an old resume that I had saved a long time ago. I was trying EVERYTHING to find it...called my brother, he mentioned to try clicking on "hidden files" to see if it was there. I did and what I found was file after file after file of porn, including A LOT more transsexual porn, along with internet dating sites for trans sexuals. He had sites book marked for places that he travelled for work! I looked at the times it was viewed and he was viewing it after I went to bed or when I was away. I confronted him again and this is what he said: It is a compulsion, he can't stop, he can only go for so long and then he HAS to look at transsexual porn. He says he has been doing ti behind my back for the last two years that we have been together and can not stop. He assured me he was not gay or bisexual. He said he has come to accept that this is a part of him and if I can not accept that, I should leave because I will not be happy........WHAT THE???!!!! We are ENGAGED! He is chosing his tranny porn over me.....he says he can't help it. What do I do???!!!! We are scheduled to see a counselor, but I can't wait that long. I don't know whether I should just pack it up and leave because he was dishonest and is chosing porn over me....or should I accept it and not over react.....or should I try to get him help because he says this is a compulsion!????? SOMEONE...please give me some advice. I love this man, I moved to a new city for him, we own a house together and we are engaged....I can't just let that all go!

Dawn80