I cheated on my wife. How do I tell her?

My wife's sister came over one day to pick up her purse that she left at our house the day before. I was just being hospitable and asked her if she wanted a drink and we got a little drunk. My wife was at some spa at the time. She said she felt sorry for me that she was out getting pampered and I was home alone. So she said if I wanted a mage...and i agreed. (stupid!) Ugh..I shouldn't of. Things got carried away and we ended up going at it. I didn't mean for it to happen, it just did. How do I tell my wife?