No, I am not mixing exposure with infection. I plainly spoke only of exposure, since that is the only factor we know condom use can effect. However, I will note that I did mention one factor which causes monogamy to promote contracting the virus, and did leave out a very important factor which make promiscuity lead to catching the virus more easily: co-inciding infections. The presence of many sexually transmitted diseases make transmission of the virus into the bloodstream much easier, such as open Herpes sores. And other sexually transmitted diseases make HIV transmission more likely to result in symptomatic AIDS, presumably by impairing the immune system.
However, that does cast a different light on the statistics. I agree that at some point in a long monogamous relationship the non HIV partner will be exposed, but given that infectivity isn't that high, (estimated between 0.005 and 0.009 for male-to-female transmission, and 0.003 and 0.001 for female-to-male transmission for a single instance of unprotected intercourse) cutting down the number of exposures makes a big difference. Which means that a condom, properly used, can prevent infection for a significant number of people.