He has a girlfriend yet he makes out with me and follows me around like a puppy? He also hides that he has a girlfriend?

So someone i know since a long time... has been intimate with me. We make out and things happen but i found out he actually has a girlfriend and is hiding it from me. Why would he do so? Why did he start making out with me or get intimate with me when he already had a girlfriend. He follows me around like a puppy which made me think he has feelings for me? But now i think he was just using me for lust. Since i found it out.. i've been distant and i do feel hurt knowing he lied to me and used me. Why would he do so in the first place?
He has a girlfriend yet he makes out with me and follows me around like a puppy? He also hides that he has a girlfriend?
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