I have been in a monogamous relationship with my bisexual girlfriend for the past six months. We’ve recently been discussing incorporating threesomes, so that she can further explore her female-female side, and we are both feeling pretty confident about doing so.
However, today, when we were talking about this, she brought up that she had been thinking about her best friend (female), who she sometimes thinks about in a physical manner. This is the first time she admitted this to me, but it was pretty obvious to me already. When I’m with them, I sense that it’s a bit more than just a friendship. They have been intimate in the past, and so her liking her friend in that manner seems more complicated and more serious than anything like a casual threesome.
Her flirting with this friend makes me feel uncomfortable, and I’m not sure how to handle it. She hasn’t done anything with her, and so I’m not really upset, I’m just uncomfortable with the situation, and I don’t know what we could do to make it better. I don’t have a solution, and me feeling uncomfortable is kind of driving her mad since she insists that I come first and that they haven’t done anything together. Any advice?
LemonVibe 
When looking to open up a relationship, it can be easier to approach someone you already know, someone you’re comfortable with.
At the same time, that can also lead to a lot of complications if this is someone one person already has some sort of attachment to.
I think you need to voice your concerns respectfully and explain that you’re not comfortable opening up your relationship with this particular woman. I think your reasons (they’ve hooked up in the past, they have more than just a casual sexual relationship, etc.) are perfectly valid causes for concern, so hopefully, she’ll understand where you’re coming from.
In many ways, if you’re going to go down this path, I think it might be more comfortable if you find someone both of you know equally well (or not at all, as it were). Otherwise, you start off with an immediate imbalance, and that can lead to a whole host of issues with comfort and trust in the relationship.