My dad is cheating on my mom

Hey
So, my dad has a job that requires a lot of business trips. Recently, he went to Boston for around a week and a half and my mom went with him. She got back on a Sunday night, I picked her up from the air port. My dad got home that Wednesday night....

That Friday, I needed to log into my dads laptop to find a file or two I had saved from awhile ago. He has never told me to not go onto his laptop. He's given me his password and stuff. He's let me go on there before so I wasn't doing anything wrong.
I log in, and I go to the internet so I can upload the things I have saved on his laptop onto an email draft and send it to myself. In the recent pages I see a craigslist posting he's been to lately and I click on it, thinking it's a car he was looking at for my mom. It was anything but. It was an add he posted in casual meetings or whatever saying he wanted to get together either Monday night or Tuesday night for a jacking off buddy and a blow job or two. Said he was married and didn't want any kissing or anal. The thing that made it even worse was that he was wanting it with another guy. (I know it was him who posted it because the page had buttons for edit post delete on there. If any of you know craigslist you know he posted that add)
I showed it to my half sister (my dad was married before, cheated on his previous wife as well, and left her when she was 8months pregnant with my sister) She's 25, and she said I should talk to him about it.

I can't bring myself to talk to him. I know I could write him a letter telling him what I know.
What should I do?
Thanks much!

heyy I noe how you feel
both of my parents are cheating on each other
but what I'm thinnking about doing is sitting both parents down and telling them how I feel

of you need anymore advice
go to my profile and read the advice that was given for me
it's under something about parents effecting my love life
I hope it helps you :]
and I'm very sorry

you should definitely talk to him,
but if you cant, maybe a letter is the best way to go.
I dont know much on this subject,
but I hope I helped a little.

If you approach your father with this, it's bound to humiliate him. If you tell your mother, it will seriously affect your parent's marriage.

Sticky situation.

Maybe you should write a letter, but instead of stating plain out that you saw the ad, you might want to play it a little safe. Maybe saying something like:

I know you have a secret, and I want you to know that I still love and respect you, but I'm afraid that your secret is going to hurt mom. Please, if you can't be honest, then stop - for us.

When he has some free time you should talk to him about it use a strong but suddle voice and ask him if he is and ask him why if he is understanding he might know that the situwation iss bringing you stress and dis comfert and might stop hopfuly usualy after you talk about things you are soppose to feel beter