This is coming pretty late, but hopefully you see it.

Just to play devil's advocate, at times I've looked at dating websites (well, really sex websites like Ashley Madison), Craigslist hookup posts, and escort service websites as fantasy and masturbation fodder.

It was never with even the slightest interest in contacting anyone or doing anything but looking. But sometimes the impersonality of traditional porn gets old, and the idea that these are (at least sometimes) real people, potentially nearby real people, adds a little something extra. I assume that's why subs like Gonewild are popular too.

I have no idea how common or uncommon that is, but for me they were essentially just a different avenue of fantasy, where others might like anything from porn to hentai to bodice-rippers to 50 Shades of Grey.

That having been said, if that was what he was doing, you'd think he would admit to it. If you'd walked in on him reading slashfic or on a porn site and about to wank it would you have been pissed, or would he have reason to think you might be pissed? If so, he could have felt just as much inclination to try to cover it up as if he had been messaging someone.

All in all, I have no idea, and I think if you can't get him to come straight with you then you should assume the worse. But I just wanted to offer an alternative opinion, since every other response here has been that no one ever has any reason to go to a dating site except to find a date.