Why does society pretend that white girls like black guys when they really actually don't?

There is no pretending. Many women are happily married or in a relationship with a black man just like many men are with black girls. Society does not say that all black men and women fancy all white men and women but there is no use to deny it happens. You don't fall in love with someone just because the color of their skin. In that case love would be a lot simpler.