Let me ask -- why don't you feel comfortable with F having another relationship? Do you think that will change in time? I know that's not what you were asking for advice on but I see it as an important issue to understand the context. In theory something can be kept at the just-sex level but you never know for sure when deeper feelings will develop even if each party doesn't intend them to.
I would say just keep it light and friendly with the girl. If you're not comfortable being super direct, you can ask her what she's looking for from her love life, just in the general sense not from F or from you in particular, and probably learn a lot from that. If flirting is happening and it feels right, I think that would be the point to ask her, "So, what do you see as the possibilities here?" (If it feels wrong, of course, that's the time to be very clear and say no thanks)
Of course, you don't have to choose to let her take the lead. If you know what you want or don't want, you can always just talk about that casually, again not even about her or F particularly, and let her draw her own conclusions, like "I'm really not open to new partners now, things are just way too stressful" or "I find being poly to be incredibly exciting because you just never know what's going to develop with the people in your life... I like to stay open to seeing where things lead."
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.