Should i confront him about this?

Hi, To make a long story short. I've been dating this guy for about 6 months. Its been going so well and we've been having alot of fun together..He's even been introducing me as his girlfriend..Well, a few days ago, he forgot to logout of his email and i saw a message he sent a few weeks ago..It was a response to a personals ad on craigslist for an "encounter". He even lied about his age in his response to this girl and said he wants to exchange pics...I was totally blindsided and my heart started racing. I feel really upset ,but i didn't let him know that i knew.. I'm in my 30's and tired of crap. Should i just confront him about it?? Why do some guys do this if they claim they care about you??? I'm really upset and don't want to be strung along.