According to Durax, Japanese have the least sex in the world. The number is almost half the americans'.
Americans Suck at Sex… According to Durex Global Survey : COED Magazine
And the use of condom is a lot more common in Japan than in the US.
In 2010, it seems the sales number of condoms in the US will probably be something like 420 million.
Condom sales experiencing solid growth
A bit old data, but in 2005, 320 millions were sold in Japan, and Japan's population is 40% of US's. If the data is correct, Japanese are 4 times more likely to use condoms when they have sex. It's not surprising the US has higher STD infection rate.
In 1998, The rate of syphilis 梅毒 infection in the states was 13.9 cases in 100,000 people. Japan, 3.6. The ones for chlamydia クラミジア were 231 and 152 respectively. Gonorrhea 淋病, 129 and 57.
STD statistics for the USA
http://homepage3.nifty.com/m-suga/std.html
US has 1.5 to 3.9 times grater STD infection rate in general to start with.
In the case of HIV, there is no data for whole US, but in 37 states, there were 42439 cases of HIV infection in 2008.
United States HIV & AIDS statistics summary
This is excluding the states like California, so overall is probably something like 80,000.
In Japan, there were 8252 cases in 2008.
http://todo-ran.com/t/kiji/11948
The rate is 26 in 10,000 for US and 7 for Japan. The disparity is a bit on the big side but still about the same as gonorrhea's case. Considering Japan is an island country, and HIV is still a foreign virus, 20% of the infection cases in Japan are of foreigners, this makes sense to me.
I was more surprised though with a data I came across while I was googling all this. In the past 10 years, condom sales, abortions by teenagers and the cases of 2 most common STDs, chlamydia and gonorrhea, are clearly going downhill.
AIDS/STI-related database Japan
It may not be a myth that Japanese young people are having less and less sex after all.