The Centers for Disease Control and Prevention CDC on their page on Male Circumcision (http://www.cdc.gov/hiv/malecircumcision/) gives this explanation to the biological plausibility between circumcision status and HIV:
"Compared with the dry external skin surface of the glans penis and
penile shaft, the inner mucosa of the foreskin has less keratinization
(deposition of fibrous protein) and a higher density of target cells for
HIV infection . Some laboratory studies have shown the foreskin is more susceptible to HIV infection than other penile tissue ,
although others have failed to show any difference in the ability of
HIV to penetrate inner compared with outer foreskin surface
The foreskin may also have greater susceptibility to traumatic
epithelial disruptions (tears) during intercourse, providing a portal of
entry for pathogens, including HIV .
In addition, the microenvironment in the preputial sac between the
unretracted foreskin and the glans penis may be conducive to viral
Finally, the presence of other sexually transmitted diseases (STDs),
which independently may be more common in uncircumcised men, increase
the risk for HIV acquisition [6-11].
What's interesting is that this ignores De witte L et al 2007, "Langerin is a natural barrier to HIV-1 transmission by Langerhans cells" which states that " Langerin is a natural barrier to HIV-1 infection, and strategies to
combat infection must enhance, preserve or, at the very least, not
interfere with Langerin expression and function."
De witte L, Nabatov A, Pion M et-al. Langerin is a natural barrier to HIV-1 transmission by Langerhans cells. Nat. Med. 2007;13 (3): 367-71. <a href="http://dx.doi.org/10.1038/nm1541">doi:10.1038/nm1541</a> - <a href="http://www.ncbi.nlm.nih.gov/pubmed/17334373">Pubmed citation</a><div class="ref_v2"></div>