I'm sorry to hear this. It's awful. This has happened to a couple women I know. When confronted, the response always veers from denial to defensive, then to accusatory (e.g. why don't you trust me, it's wrong to snoop, - trying to make out that her finding the profile is worse than him having one,) then maybe trying to justify or at least putting hands up and admitting it.
In one case though, it transpires her boyfriend's teenage daughter put up a profile for him "as a laugh" (and because she wasn't keen on my friend at this stage.) Someone saw the profile and tipped off my friend who was livid. He seemed baffled when she told him but had an inkling his daughter was involved but a few hints she'd dropped. He rang her and she confessed.
I think the latter scenario is rare, very rare. His response to being confronted was also totally different. He did say he didn't know anything about it - which was true, but didn't get defensive and insisted understood that she must think he was an absolute shit. He didn't try to invalidate her anger, didn't make a song and dance about her snooping and didn't try to make her think it was her fault.
Okay, you're not going to have any peace of mind until you know what the score is and can make an informed decision what to do. As Flannelle says, collect the evidence. Keep it on a couple memory sticks or take photos of the screen or whatever. I wouldn't suggest investigating further as you could feel even more hurt if you see other things.
You will have to tell him what you've found. If he reacts defensively, accusing you of being in the wrong, etc., it's not a good sign. If he tries to justify it (e.g. it was just a lark, oh, it's an old profile I just checked in on, it was only chat, I wasn't planning to meet her or anything, whatever,) beware. If he tries to blame you of make you feel bad for snooping, that's not good either.
You may need to make some tricky decisions about your marriage fairly soon. Do you have RL folks you can call upon for support? Take care.