How do I tell him no???

I need so much help ladies! How do I tell my man no??? So.... a week or two back my boyfriend convinced me to have a foursome and it was okay... don't get me wrong I enjoyed all he attention from the guys but when I was with the girl, I felt absolutely no connection, there wasn't a spark... I felt nothing. I didn't enjoy it at all, just thinking about it upsets me because I faked it all and he thinks I'm into that kind of stuff. The only reason I faked it was so I didn't make the girl feel bad! But now my boyfriend wants to have a threesome with her and I am so against it. I don't want him to talk to her, fuck her or to fuck me. And I don't know how to tell him without sounding mean or like a bitch.... because he knows I live by the quote "you don't know if you'll like it until you try it" but I'm so against that... maybe it's just the girl because I remember her from years ago and she was so mean to me but that's a different story...