Most Helpful Guy
It's easier than you think. Guys get insecure easily. He would hate you like anything, in case he finds out that you cheat on him and make fun of his p*nis size when you are with that other guy (u don't have to actually do it, ask your guy friends to act along)! You can show this to him in many ways, like text chats with that guy, talking about it on speaker phone (by mistake) while your fiancee is around, or plain old getting caught with!
If you don't wanna spoil your repo, then spread rumors that he is a cheating polygamous person. Tell some of your open minded female friends to post on fb about her one-night stands with him! And make sure that he knows that you are doing all this, so that he starts hating you extremely.
Most Helpful Girl
Where are you from? Sounds like an arranged marriage. How awful.
I think that saying you've slept with a couple guys would probably do the job.
Feel free to go in full detail.