Harvey, you say she's a "female friend" ... as in a casual relationship I'll assume. Have you guys even been intimate with each other yet? And if so, for how long? It just sounds like you sprung something on her when her focus was on being with you. You know, "timing" is everything with these kinds of discussions. Maybe she has (unbeknowist to you) desires that the relationship she has with
you might become more serious and committal. OR, what you said, and/
or how you said it, shocked her and turned her off. I know many guys think that "white women going black" or "cuckolding" appear mainstream (that practically everyone is doing it), but they definitely are NOT mainstream; BUT, they are growing in popularity. Another thought ... if you guys are just "friends" and have no desire to be intimate in the future, she's probably thinking you crossed the line into her personal business. But I'm thinking that's probably not it.
If you really care for this girl's friendship, you'll call her on the phone, and give her one of those (hat in hand) serious apologies for being so "bone headed" as to assume she'd even be up to discussing this kind of topic or any other sexual fantasy with you. If she won't take/return your calls, send her a sweet letter with some pink (not read) roses. Women love guys to be humble and show sensitivity. Your mission now is to salvage the relationship, patch up the damage you did, and start back slowly. This is YOUR chance to show her a side of you that she maybe has never seen ... don't BLOW IT! It sounds like maybe this friendship isn't ready for this much openess quite yet.