I kissed a guy with a girlfriend last night
How did you happen to kiss a guy with a girlfriend?
I had a chance and took it.
Sometimes you cannot help who you are attracted to but you should avoid kissing people you know are in a relationship. It will cause you too many problems. You should seek people who are single to date.
I'm just trying to have fun.
You can have fun with other single people. If you date someone who is in a relationship you will  not be a priority for them, their partner will be.