Due to increased vaginal friction, of course.
Lancet. 2009 Jul 18;374(9685):229-37. doi: 10.1016/S0140-6736(09)60998-3.
Circumcision in HIV-infected men and its effect on HIV transmission to female partners in Rakai, Uganda: a randomised controlled trial.
Observational studies have reported an association between male circumcision and reduced risk of HIV infection in female partners. We assessed whether circumcision in HIV-infected men would reduce transmission of the virus to female sexual partners. …
The trial was stopped early because of futility. 92 couples in the intervention group and 67 couples in the control group were included in the modified ITT analysis. 17 (18%) women in the intervention group and eight (12%) women in the control group acquired HIV during follow-up (p=0.36). Cumulative probabilities of female HIV infection at 24 months were 21.7% (95% CI 12.7-33.4) in the intervention group and 13.4% (6.7-25.8) in the control group (adjusted hazard ratio 1.49, 95% CI 0.62-3.57; p=0.368).