OK, so here's the deal.
Most of the people who are telling you not to do this are not objecting on moral grounds -- they are objecting on PRACTICAL grounds. It's very simple: The vast majority of the time that people in a monogamous relationship start having sex with other people, whether condoned by both partners ("swinging") or not ("cheating"), the relationship fails. Many posters have shared their own anecdotal evidence, but I'm sure you would hear the same thing if you asked a much larger set of people. It's just how it works with human beings, no matter how we wish things might be otherwise.
So, there is an extremely high probability that your current relationship is already doomed since this isn't the first time you have involved others in the bedroom with you and your GF and she obviously enjoyed the experience enough to want to do something like it again. There are other risk factors that drive the probability even higher that things won't end well: your age, the distance, the fact that you're in college and people change a LOT in college. For what it's worth, the only couples I know who survived this situation did it ONCE, realized instantly that they had barely survived a game of Russian roulette with their feelings, and NEVER did it again. And, more than that -- not only did they not do it again, they truly didn't WANT to do it again.
Claiming that every person is unique and every relationship is different is a truism -- it isn't a valid rationale for believing that somehow, you will be the 0.01% (or whatever) that survive this situation intact. Lhadatt is dead-on -- why in the world did you create this thread if you didn't want to listen? You're picking and choosing what you want to hear: "Oh, it's still OK, but maybe I should be there." No, dude -- IT'S NOT OK, PERIOD. Unless you recognize that things are doomed and have changed your stance to be, "well, I should enjoy things while they last," which is where mosaic was coming from. The way he worded it was harsh, but he's basically accurate as well. The simple fact is that most of your dating life is going to be a series of failures, but the purpose of that sequence is to a) have some crazy-ass fun and b) learn about who you are and what you're really looking for. It's training to eventually get things right with a quality girl (or guy, if the ladies are reading this) since we do NOT come into this world knowing shit about how to make someone happy over the long run, and you don't want to find yourself in the situation of losing someone great because you didn't go through the necessary screw-ups when you were supposed to.
Finally, I'm not some kind of genius about dating. I'm not that attractive or smooth, and I'm not a psychologist by training. But there are three kinds of people in the world -- those who burn themselves on a hot stove repeatedly, those who burn themselves once, and those who watch the other two kinds burning themselves and figure out to stay the fuck away from the stove when it's hot. I try to be in that last category as much as I can, and I tell others about where I got all those little scars on my hand to do my part.
So take it or leave it, man, but at least recognize that a lot of people made a gift of their time and hard-earned wisdom in this thread...and in a lot of threads on GAF. They don't know you, they don't have any kind of stake in what happens to you, and yet for some reason, they take the time to share some shit to try and help you out. You very well might have to go burn yourself, but at least show some respect.