In Nancy Padian's 10 year study, none of the over 100 HIV- monogamous partners of HIV+ subjects seroconverted to HIV+ during the study. They were counseled to use condoms, but it is believed that they often did not.
[Ref: Padian NS, Shiboski SC, Glass SO, Vittinghoff E. 1997. Heterosexual transmission of human immunodeficiency virus (HIV) in Northern California: results from a ten-year study. Am J Epidemiol 146, 350-357]
Aids Truth says the study was misinterpreted by denialists. It says that the rate for male to female transmission was 0.01 to 0.32 % in the developed world. That is once per 300 to 1000 sexual contacts. It's vastly too low to account for the "spread by infection" presumed to have occurred before it was noticed.