People have heated debates on this subject but rarely does anyone think to present any statistics to back up their opinion.
Studies show that the risk of HIV transmission is reduced by a factor of about 5 by consistent use of condoms, e.g.
http://www.ncbi.nlm.nih.gov/pubmed/11869658.
The risk of acquiring HIV could be roughly approximated as a product of three factors: number of sexual acts x probability partner is HIV positive x reduction factor for condom use. As noted, the reduction factor for condom use appears to be about 0.2, which is not particularly low. The very low incidence of HIV in heterosexuals with numerous sex partners would appear to the very low probability that the partner is HIV positive in the first place. This is not to say that condoms do not make a useful degree of risk reduction but they are not as effective as people perceive them to be. We get away with multiple partner sex not so much due to condoms but to the low prevalence of HIV in heterosexuals in the USA in the first place. In parts of Africa, where HIV is widespread in heterosexuals, sex with or without condoms would be risky (though if you are going to do it, you are obviously better off with a condom).
The converse point is that if the prevalence of HIV in the pool of people with whom you are having sex is very low, then not using condoms will increase your risk by a factor of 5, but 5 times a very low risk is still a very low risk.