2 months ago I met a guy from Spain that was in my country just for a short time. I had just broken up with my boyfriend 1 month ago and I wanted a one night stand (i know it sounds bad and now I regret it!) so this Spanish guy and me had some dates and then we had sex.
Now I found him on facebook (he told me that he didn't have one ) and i see that he has had a girlfriend for 2 years!! I feel awful but I didn't have idea that he had a girlfriend.
I can understand you are feeling guilty. However I would say it's the guy's fault because he already had a girlfriend when he had sex with you and so that's cheating for him, however you had sex with him after you broke up with your boyfriend so when you had sex with him you were officially single, so at least you have a reason to feel less guilty.
However you said you had some dates with the Spanish guy but then it looks like you never made an attempt to know the details about him, like you should have asked him if he had a girlfriend or not, that's a basic question you should have asked before you had sex with him, and in that way you could have stopped yourself and also him from making a big mistake, but for some reason you didn't do that, so you are also at fault.
However as I said you can feel less guilt compared to him.
What you can do now? well you should forgive yourself for what you did and learn from your mistakes and next time think 10 times before you do something.
Contact his girlfriend, tell her what happened. Apologize, make it clear you had NO IDEA he had a girlfriend, and never would have hooked up if you did know, because while you may have been looking for a one night stand, you were NOT looking to hurt anyone else.
She'll be mad, but if she has any sanity, she won't be mad at YOU. He betrayed her.
It's not your fault. There's nothing wrong with one night stands and also you didn't know he had a girlfriend. He takes %100 of the blame. If you can reach his girlfriend, you can do the moral thing and tell her.
Usually I'd call those who sleep with taken people sluts, but in your case you're a victim too since you didn't know about it. You can either act like nothing happened, or find a way to alert is girlfriend about him being a skank.