Why did he ask me to keep it a secret?

confusedgirl18
I was hooking up with a guy a couple months ago but we both went away for the summer. Since then, I've become good friends with one of his guy friends (who is casually dating someone). Last night he was in town and we hooked up again but he asked me not to tell the guy friend about it.

I wasn't planning on telling, but why would he want me to keep it a secret? He said his friend would be upset if he found out but I don't understand why?
Why did he ask me to keep it a secret?
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