It has nothing to do with STDs.
For some reason, it's a common problem in Japan and is called "houkei(包茎)."
Pick up any men's magazine and you'll see numerous ads from clinics offering to correct the problem.
As SG pointed out, the cock can be washed when flaccid.
I've never encountered an unclean cock in Japan, and rarely in Thailand, no matter whether or not the foreskin retracts.