Being not black I can just suppose if i would find it offensive, or imagine a similar treatment to my ethhnic group (Italian): men wanting an Italian to fuck their wives, and women wanting an Italian just for sex.
At first sight it might even seem a compliment, but I have to agree it proposes "racial" if not "racist" stereothypes.
On my opinion, however, reading some lines cannot discolse racism on a message, such it just depends on many other factors: do those people know that those are just stereothypes and that God created all of us to have the same rights? Outside the bedroom how would they interact with a black person? would they accept they son/daughter to marry a black person? The comments about virility are just what we read or do they suppose that black persons have less intellectual skills? is there any irony in the message?
I mean, most of us know that penis size has less importance than what we pretend here, do those people write with the same spirit?
Without answers to so many questions, my opinion would be pure theory.