First of all, understand that she's lying to save your feelings when she says she likes your small cock. Maybe she doesn't like black guys, but she'll definitely appreciate a solid fucking.
As to convincing her - you have to understand where her reluctance is coming from. Typically, female reluctance to open their relationships (and their legs) to other men is rooted in a few basic factors.
1) She married you because you were the best she thought she could get. To make that decision she had to convince herself you were an alpha male. That may involve some level of denial or self-deception on her part, which is intrinsically hard to overcome.
2) She wants to feel valued and desired by you. If you're willing to share her, according to her internal belief system, you must not value and desire her. She doesn't want to believe this either.
Squaring these two circles is difficult, to say the least. Best of luck.
S