Would you be worried if your boyfriend hangs out with his ex fling alone?

I was hooking up with this guy in the summer and now he has a girlfriend. I stopped texting him after I found out he has a girlfriend. But now were hanging out again. He takes me out to dinner and pays for everything and we chill at his apartment prob stay their around 1 am and yes were alone. So girls would you be worried if your boyfriend is doing this behind your back? And guys, why would you do this when you have a gf?

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