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listen to your gut. It's SCREAMING at you. Not only is it trying to tell you that something really, really stinks about the fact that he's willing to risk hurting his "live-in girlfriend" just to check things out with someone he only knows in cyberspace, but it's even telling you that you shouldn't have moved in with anyone who can't even choke out "I love you," no matter how half-hearted.
There is a price that you will pay if you stay with someone who isn't really that into you. And the price is that you will slowly, almost invisibly but inevitably, become convinced that you aren't worthy of being truly loved.
Trust me. I've done it. I dumped him after four years, but it took another dozen before I was able to address the underlying chaos it caused in my view of myself.
Run. Don't look back. Better to spend the rest of your life with hope and alone, than hopeless and with him in whatever lame and compromised form that might involve.