You really need to use the right words in your own mind about this, too. If you were seeing a guy who had a girlfriend, then you weren't his girlfriend. That means he isn't your EX.
If you want to understand him before you'll stop obsessing over it, that's unfortunate. People who don't lie much can't understand people who lie a lot. Period.
He doesn't love you or his girlfriend, at least not like you mean. Fr_Chuck is right, love is just a feeling when you're first with someone. It's a motivator, but it's not the "love" you think of that never ends.
You're going to experience a lot of false "love" thrown at you in life, get used to it. Learn to convert it in your mind to what it is...desire. REAL loves kicks in when time has proven there is a real commitment, when the person you love is loving you back by putting your needs above their own need to misbehave.