I Am Confused About My Feelings

I have feelings for this one guy I know, but he has a girlfriend.
Whom he has been with for 4yrs or so.


I did not flirt with him, btw
We were friends, and then he kissed me??
Why did he kiss me if he loves his girlfriend??

Am I a terrible person for kissing him back??
We haven't spoken since the kiss, because we don't see each other that often.
I need advice, what do I do now??