Start off by talking to her about it over
Some drinks or while having sex. Maybe she can fuck a guy who is her ideal man , then next you can have your bbc fantasy when she’s more into it.
My wife is super conservative and shy and I ended up bringing it up while having some drinks and playing with her. Started off asking about previous sex partners and when she got into it , I admitted I wanted to see her with another man. Haven’t done it forreal yet - but she likes to talk...