I messed up bad.

I've been with Mark for five years. Five years! And last night... last night I kissed David. David from work. It wasn't even like, some passionate thing. It was just... there. He walked me to my car, and we were talking, and then boom. We kissed. And now I feel like the scum of the earth. Mark doesn't deserve this. He's the best guy I know. I don't even know why I did it. I think... maybe I'm bored? Or maybe I'm just a terrible person. I haven't told Mark. I don't know if I should. God, I hate myself right now. Please tell me I'm not the only one who's ever done something this stupid.