...but that they are simply not the right variables. We don’t select our partners from a random pool where 1 in 250 is HIV-positive. There are a vast number of subgroups from which most of us choose not to seek partners: the retarded, the mentally ill, the physically handicapped, the congenitally deformed. Since the incidence of sex among these subgroups is thus artificially low, the incidence of HIV is also artificially low, driving the incidence across the population drastically downward.

Which basically says that the two variables are not independent. If we selected partners randomly then the two variables would be independent. But we don't as you note above.

Furthermore, we don't select a new partner for each and every sexual contact: we tend to keep the same one as long as possible before switching. The argument is just plain broken out of the gate, no matter what the numbers happen to be.