I may be wrong but, I think the reason circumsised men have lower rates of std's is because the groups that have it done - relegious or higher social / economic class, tend to have less sexual partners and/or practice safe sex more.
If you look at the links I posted you'll see that most of the studies were conducted is sub Saharan Africa. There no inference from any other cohorts.
It's nothing to do with cutting a bit of skin off, but the myth is kept going in America. I mean think about it, hiv is in the fluid how would cutting the skin stop the spread.
The studies I cited did't test for a mechanism of action. The effect is quite robust and not in anyway a "myth". Here's what researchers think the MOA's are:
Study Shows Circumcised Men Have Less Risk of Herpes, Genital Wart/Cancer Virus
How can circumcision prevent STDs? In at least three ways:
When the foreskin is removed, the skin covering the head of the penis becomes tougher. That may protect against "microtears" during sex that can provide a point of entry for germs.
The mucosal lining of the foreskin may allow germs to penetrate to underlying skin cells.
After sex, the foreskin may prolong the time that tender skin is exposed to germs.
Circumcised men may be protecting their sex partners as well as themselves, suggest University of Washington researchers Matthew R. Golden, MD, MPH, and Judith N. Wasserheit, MD, MPH, in an editorial accompanying the Tobian study.
Data from earlier studies indicate that monogamous women with circumcised sex partners are only half as likely to get cervical cancer as are women with uncircumcised sex partners. And the Tobian study shows that circumcision cuts the risk of HPV, the virus that can cause cervical cancer