My FWB's Best friend Hitting On Me - How do I turn him down w/out drama?

Juz-A-Friend
I've been in a "crazy" relationship with my guy for almost 5 years.. We go on dates, hang-out most of the time and are into physical stuff but we've not labeled our relationship. I would say it's sort of an "open relationship" - so open that it's only the both of us who knew that there's "something" going on between us. Anyway, I guess even his bestfriend just knows me as one of his close friends. This is where the issue began. A couple of days ago - I receive a facebook msg from my FWB's bestfriend... these are the exact words:.."we should chill-out at (my FWB's name)'s place sometimes and have couple of beers and just hang-out there.. what do you say to that?"... and me being nice knowing that he was my FWB's bestfriend... replied to him back - "yes it would be nice to do that.. what do you say tomorrow night.. as I was planning to come-over (my FWB's name)'s place to watch a DVD movie?"... And then he msgd me back this " yeah, we can do that tomorrow night... and if (my FWB's name) doesn't have time.. we can hang-out at his place just you and me... I will ask him" -- This last msg threw me off.. for one this guy is married. How should I turn him down, without making any drama out of it? Should I tell my FWB about it and asked him to deal with his bestfriend? Or should I just send the bestfriend msg back telling him - it's not a good idea just me and him especially in my FWB's place. NEED ADVICE PRETTY BAD. Thanks.
Updates
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Thanks for the advices from 2 very nice women, McRoo and missnicole.I talked to my FWB earlier and told him the issue with his bestfriend. Well, I didn't like how he reacted at first and I'm not comfortable sharing that here..
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Although, in the end of my conversation with my FWB when I told him I'm not cool with his Boyfriend hitting on me. He told me to just ignore his besftriend and don't answer him back anymore and he even told me to block his besftriend from my facebook.
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OOopps... typo error... I meant "bestfriend hitting on me"... and not boyfriend... LOL!
My FWB's Best friend Hitting On Me - How do I turn him down w/out drama?
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