HIV transmission does NOT have a constant level of risk per exposure.  Some people are genetically resistant (e.g. CCR5_Δ32#CCR5-.CE.9432) as potential recipients, and some people are likely not to be very infectious even if infected (e.g. HLA B*57).  In addition, acutely infected people are the most infectious, and people with genital ulcer disease are likely to be both high transmitters and particularly susceptible.  So, the risk depends dramatically on many variables.  In addition, it's known that the cumulative risk approaches only about 40% (in discordant couples), and the uninfected partners tend to show evidence of cellular immunity to HIV (suggesting exposure).