I have been seeing a girl for 3 years now about a year and a half ago i got blackout drunk and had sex with some chick and told her the next day what happened she was upset but didn't want to break up now we are engaged and not to long ago she caught me looking for casual sex on craigslist. she is upset but still wants to get married. now i don't mean to be bad though i am a sex addict i work hard to not look at other women and avoid temptation but this can not be a healthy relationship she will probably be suspicious of me forever and she will dangle this over my head forever i still love her but i think under the circumstances we have too much bad air already in the relationship to make it work. is this normal

I'm not going to sit here and pass judgement on you, but take my advice.
Leave her. She obviously does not have her head in the right place and you should not have gotten into a relationship with her to begin with. If you are a sex addict and you can not control your urges then you have no need to BE in a committed relationship. You will only end up hurting both you and herself and if you care about her at all then you will do what is best for everyone and leave her.
If you honestly just have such powerful urges for multiple partners, perhaps you should consider an open or polyamorous relationship with someone who would be into that kind of thing.
I used to think that "sex addiction" as a bogus excuse for horny guys, but I also thought that "anxeity disorder" was bogus too untill I got it! I wont judge you because I recently have had to face alot of realities about disorders myself. If you don't either gain control of it, or seek help in doing so, you will never have a working relationship. Just my thoughts
Perhaps she has dreams of walking down the aisle in white and won't let you spoil that 'big day' by breaking off the engagement. She probably figures that she'll deal with you AFTER her day!! Boy, will you be in for it!!
However, you are right - the marriage will not work but at least she will have her wedding to look back on and perhaps half of all you own to console her ;o)
Leave this idiot at once. No-one loves a walkover!