For men: Why would a man be on a dating site talking to women if he didn't intend to cheat?

I found out through the history in the computer that my significant other was on the yahoo personals corresponding with a y blonde woman and inviting her to a baseball game. During this time, he was away from me for a week. He claims that he was just bored and had no intention of going through with it and that he'd never do that to me. I asked him why he had an ad on the dating site sinse we've been together for 5 years now. He claimed that it was an old ad that he still gets mail from and that he just looked out of curiousity. He said that it meant nothing and that I have nothing to worry about. Should I give him the benefit of the doubt or is he just being a lying/possibly cheating rat? Opinions please.