Stl
I hate to break it to you but if you two are a couple then the moment she has sex with another man you are a cuckold. She may be more worldly than you think and possibly more than you are... These women start much younger than I ever did at your age. A few questions; are you two a couple? Is the guy she wants your ex stepdad? If not is the guy Black?
Explain that you don't want to lose her but you also want her to experience those pleasures. Tell her the thought of her sexy body in the throws of passion with an older Black man is very erotic, any Black man would be an erotic and a scene you'd love to see. All this is cuckolding at it's core but I assume that you want to be involved with any sexual pleasures... Tell her that seeing her with another man, especially a Black man would turn you on, excite you more than actually you doing it with another woman...
All this establishes that white women and Black men are potent sexual scenarios and that you are okay with her doing that... That you are okay with her having sex with other men, especially Black men... And that you are not doing this so you can have other women but you want her to enjoy the pleasures of other men... Last but not least tell her you love her and that none of this will make you love her less.
Remember the idea is to SHARE ALL THIS WITH HER AND NOT DRIVE HER AWAY... Doesn't matter if she is already fucking the Black Daddy, you want her to bring it home to you.