Please help need advice

I went out for dinner with someone who I thought was just a friend. We met up with more friends for karoke later that night and we were all dancing. He kissed me and I pulled away. I have a bf and was scared to tell him so I didn’t. The guy asked me out again and then my bf found out about all of this.

How do I tell the guy that I don’t want to go out again without coming off as a bitch given he thought we were out on a date that night.