And my story is going to sound kind of horrible, but note I'm being honest, and this was nearly two decades ago, and I am now a very monogamous man with a very happy marriage and kids (do you know how fucking rare that is?!?!).
Once back in University. Basically, she was a religious sort of girl, very nice... and I am quite the opposite sort of guy - irreligious, and generally a casual dater.
My ex-girlfriend - I met few years before I started dating her, she was in one of my classes, and we had a very flirty sort of friendship. I didn't know she was religious at the time.I was also in a relationship at the time. Anyway, we fell out of touch, and years later I met with her again, and this time I was single, I asked her out, and we clicked. Anyway, she was super shy about sex, and sex is very important to me in any romantic relationship. She was saving herself for marriage, we fooled around in other ways... but in time we began seeing each other less and less - it went from every day to every 3 or 4 days. Being religious, and from a religious family, her parents still kind of controlled her life, she lived with them, and was "not allowed out" past certain times despite being a woman in her twenties.
Then my birthday party hit, and she went home early. Of course, being one of my parties, it went very late. Long story short; at some point after she left the party due to it getting a bit late; I was a bit miffed about that! Then at some point people began taking off their clothes randomly, I think some dude took off his shirt, and then shortly after someone took of their pants, and it spread like a California wildfire! The party became a bit of an exhibitionist scene... everyone naked or in their underwear. People began making out and doing other sexual things, and I ended up in bed with two younger women in their first or second year of university; both quite fit, both dolled up, both horny. One a blonde, the other dyed pink hair. I'll note that these two weren't random girls either, I had known them through parties and going out for a while now, so it wasn't completely random. To be fair, both of them were cheating too; and they were also both dating kind of shy-non-party type guys from their classes and were in (probably) equally as unsatisfying relationships.
In the end, the reason is that the relationship was on its way out, I knew it, and I had a fairly fantastic opportunity. I will say I was self-inflicted morally hurt when I sobered up, I'm not a total cunt!
I don't say this is a particularly bad move on your part. If you knew your relationship was over, and you had an opportunity to do something that is generally pretty hard to come across, I'd say you were right to take it.
Now, if you continued dating that other girl, and repeated the same behavior again shortly after the first time, I'd say that's pretty shity. But, if the extent of it is contained in your comment, you shouldn't feel too bad about that. On the moral failures scale, I rank it somewhere around stealing $5 from your mom's purse when you were a kid..