Maybe she could've been nicer about it, but I'm not gonna lie, I think you would've caught me looking at you with complete disgust too.
With sex, sure we can get hot hot and steamy, when it's about YOU and the guy you're talking to ONLY. It's an intimate thing that you're sharing together. It makes her feel special that she's the only one. When you bring up the idea of a threesome, it makes us feel unappreciated. Like we're not enough, that you need more than just us. And bringing up her best friend is even worse! Girls are extremely competitive, especially with the girls they're the closest to. So finding out you find her best friend attractive enough to sleep with.major ouch. What if she asked to have a threesome with you and your best friend?
LOL! You silly boys! A woman wants to feel like the sexiest thing in the world. When you add another woman to the picture in her mind you are saying she, by herself, isn't good enough. No way is a normal woman going to go that. No one wants to hear they are not good enough. Now I know in your head your saying, "That's not what I meant at all!" but there is no way you are going to make her see it your way. ever. If things are going well for god's sake don't mention adding additional women. If she is up for it she will bring it up. Good luck man.