yes, I think it's cheating if he doesn't want you to know about it. He's secretly plotting or flirting with having a liason with someone else - that's cheating.
It's NOT cool, but if it makes you feel any better, he's not going to hookup with a girl that way. (Not unless he wants to pay her, anyway)
I've always heard those ads are not at all what they appear to be. Real live women apparently don't post personal ads there. The ads in question are either electronic spam generators, professional prostitutes who will meet with your boyfriend only for a fee, or men who hope your boyfriend will just decide to go for it anyway when he discovers his "date" has a 5-o'clock shadow.
Tell him to get a clue and pay attention to YOU instead before it's too late. Save craigslist for what it was meant for: apartment listings and garage sales.
Don't care if he met up or spoke to or anything, just for him to move the mouse over there is bad enough. It's my partner, as if she is the only one on earth, period. I go on some sites to move certain digital formats, from one said pc to mine, and there are this woman on the sides of the screen, even though I'm single I don't look and actually cover the pics until I'm finished my search, how much more should it be it I were with someone?
If he answers the ad he is meeting them
Cheating is cheating! Doesn't matter if it's in person or on-line. Just the fact that he responds to the ads is enough to show that he is not completely invested in your relationship.
Anything done without your knowledge .... has to be cheating!