I'm gonna go with intent here. Unless he's an investigative journalist writing a story on online dating, or a private detective trying to catch cheaters in the act, he's probably looking for a little side action. The fact that he went for an "adult" dating site is a pretty good indication that he's not looking for a new Words With Friends pal. In fact, I'd take the "adult" part as a sign he's specifically looking for a discrete affair. It's not like he signed up for eHarmony.

After six years, he should be looking for a ring online or tickets to Aruba for your destination wedding. What he shouldn't do is troll dating sites. That's the sign of a guy who is looking for a way out of the relationship. Because a guy in a relationship doesn't go to the trouble of making a profile on dating site unless he has a goal of finding a one-night stand or a mistress.  You don't just "see what's out there" when you're in a six-year relationship. He knows what's out there-- someone to cheat on his girlfriend of six years with.

You should discuss this immediately. And by "discuss," I mean dump him like yesterday's garbage. Sure, he hasn't cheated yet. But the intent-- and the disrespect he's showing you--is enough. It's emotional cheating. Why invest even more time in a guy who has clearly checked out of the relationship a long time ago? After six years, if he's making online dating profiles instead of proposing, that's a clear sign he no longer wants to be with you. At least you found out now before he slept with some random woman behind your back.