I think some of the advice given is a load of shit to be honest. You're 22 years old. If you're curious about what else is out there now, then more than likely, you're going to be curious after you say "I do." I swear, I don't understand why people decide to get married and haven't truly lived. That's just fucking crazy- get all of that shit out of your system before you decide to tie the knot. You wanna know why your fiance' gets pissed when you talk about it? Because you tell her how lucky you are to have her and how you want to be with her, and then slap her in the face with "I want to see what else is out there." There's no way you can justify that. You can't always have your cake and eat it too, and if she's willing to bend on this, then she's one hell of a woman. I know a lot of women who would have told you to go exploring and have your fun-but they wouldn't be there when you're ready to come home.
You're about to make a real life decision that doesn't just impact your life, but someone else's. Right now, there's a lot of "me, me, me," instead of what someone's who's engaged to be marrying should say, which is "we, we, we." Get it through your head- if she's upset about it, then more than likely, she doesn't want you to do it. And if you're as lucky to have her as you say you are, then the only woman in the world that should matter is her.
This is exactly why I say you often have to treat people like cars- you need to test drive before you get locked into contracts and agreements. Which brings me to something else. You've been engaged for 8 months(and I'm guessing that you were together for at least a year before marriage was even thought of). If you haven't set a date, or plan on getting married any time soon (your words, not mine), then what the hell are you doing? You may care for her emotionally and you believe that she's one for you, but what you're saying speaks otherwise. Before any arrangements are even made, you need to think about your life. You have a lot of men on this site who had curiosities that they wanted to explore and never did and got married. Now they're jerking-off with each other and women on skype, yahoo, and in the video chatroom because that's the closest that they'll ever allow themselves to get before stepping outside their marriages.
You two can definitely make it through this if you decide to let it go or get it out of your system. But when it comes to sex, you don't want to stop at just 2 encounters.