My girlfriend (let's call her Claire) shares a flat with a couple (let's call them Bob and Alice). Bob is a close friend of mine, and I get along pretty well with Alice. The three of them used to sleep all together, in an open relationship, and things were fine (that was before the bonds between Alice and Bob tightened and Claire and I started our relationship).
Bob was away for the weekend, and one night, Claire, Alice and I went to have some drinks. Things were going smoothly up until the moment Alice told us that it would be cool if the three of us could sleep together that night, and that she sent a text to her boyfriend to ask if he was cool with that. The answer was no.
Even if my girlfriend was cool with that, I wasn't, because Alice is the girlfriend of a close friend, and I have the feeling that it isn't the right thing to do regarding Bob. I couldn't reveal my point of view at that moment because the text had already been sent. I now fear that Bob could hold some grudge, or that those events could alter the relationship Bob and I have.
How can I approach Bob to clarify things, and explain to him I wasn't cool with all that was said and that I didn't play any role in all of this?