Re: Circumcision in Japan
It looks like the circumcision rate in Japan is probably around 1%, so I think this girl is very misinformed. It's very common in male or female circumcising cultures to assume everyone else does it too, but 80% of the world don't cut their children's penises. Another possibility, is that people can get confused about the terminology. Also, people can even be confused if they happen to have only ever seen erect intact penises from a distance. The erect intact penis looks practically identical (from a distance) whether circumcised or not. On the intact penis, the glans is exposed when its erect. (The way the penis naturally works, the glans is only visible to indicate sexual excitement. I think that's why some think the cut penis looks better.) The intact penis is usually only covered by the foreskin when it's flaccid (like everything like this, there's a lot of variation). Of course up close you can see that the skin looks really different. It's fresh and healthy on the intact penis while the texture is rough on the circumcised penis.
And to answer your second question, studies are very divided on that question of whether circumcision might reduce any STDs. The highly referenced studies in Africa suffered from severe methodological flaws especially the fact that it seems the participants may have been acquiring HIV non-sexually (from improper medical care). Most studies in the US and elsewhere have found no disease transmission benefit for circumcision. In fact many have found only negative effects of circumcision. A study in the Journal of Sexual Medicine (2012) among Caribbean men states "compared with uncircumcised men, [circumcised men] have accumulated larger numbers of STI in their lifetime, have higher rates of previous diagnosis of warts, and were more likely to have HIV infection." The only randomized controlled trial into male-to-female transmission found that the circumcised men were 62% MORE likely to infect their partners with HIV than the intact men were.
The whole debate about the value of circumcision should be ended by considering the apparently obvious fact that extra sensitivity the foreskin provides makes condoms more appealing to men.
That I know of no study has confirmed this observation, but among many men I know, I have noticed that the men with foreskin are MUCH more likely to choose to practice safe sex. Additionally, the only men I know who totally refuse to use condoms are circumcised (and think all men should be cut like them).
Personally I don't think parents should be permitted to make any irreversible body modifications to their children. Circumcision is a lot like castration, footbinding, breast ironing, or involuntary gender reassignment; it's wrong to modify the body of another human being. If people want their bodies modified, they can have it done to themselves when they're adults.