Cheating husband....possibly?

I found a personals ad my husband placed on an adult website. So I confronted him about the ad. He said he’s never heard of it and has no idea how the ad was created. He said maybe the website stole some info from out couples ad we had up awhile ago (another story) from another personals site. Needless to say, I didn’t believe him and I told him so. He said “I don’t care. It’s the truth”. What do I do? He’s obviously lying but I don’t know if I want to leave him because of this. How do I find out the truth? I’ve already tried emailing him as someone else from the site. He’s not a paying member so he can’t respond. I’m torn. Is this considered cheating? I would love some male advice (serious advice)