If I remember correctly, long time ago, I think in Japan, I went to a hotel. I felt hot so I tried to change the A/C to around 18 c, and it's still hot, so I just turn it off and opened the window.

Actually we are talking about why there are so few cases of HIV in Japan, and we should probably steer back to that subject.

OK, back to the topic.

Compare to other nation, how active the Japanese are in sexuality? I heard that nowaday they tend to have less sexual relation/having their own family which is reflected in low birth rate. Is this the factor of low HIV rate too?

This is something I'm actually curious about too, along with why Japan's crime rate is so low compared to other industrialized countries.

I think maybe in japan there is a very disproportionate amount of nerds/otaku's that don't have sex and don't have any ambition to commit any crimes. There probably aren't anywhere near as many whores in Japan either. In america it is very easy to have one night stands with different women every week. I'm actually surprised the HIV/STD rate isn't higher here.

It takes a ridiculous amount of time to become efficient with an abacus! It might as well be a qualification!

I had to learn to use it in primary school in Algeria, but if I had to choose between a Calculator and an Abacus, it'd have to be the Calculator! The only thing the Abacus did for me was improve my mental arithmetic. By the time I moved to England, my mental arithmetic was the only advantage I had! So, I agree with "If it ain't broke, don't fix it", when it comes to education, because it's terrible seeing little kids using a calculator and not having a clue how to use their brain, but otherwise, at the office, the system is pretty broke if you have to look for an expert abacus user. (Though to be honest, Excel is the new tool)

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.

As for abacus, there is an "art" of using abacuses called Syuzan 珠算 in Japan (oh how they like to make everything an art... Calligraphy, Flower arrangement, Tea party, now even Abacus) and the experts of that art do really complicated calculations in their head even without an abacus (at the expert level, it's faster to use an imaginary abacus in their head than using a physical one), and there are even competitions where they compete in Syuzan skills. You can't really compare it to a calculator. You won't be good at calculating stuff no matter how much you use it.

This is in Japanese but it's an interested read about Syuzan's future and it's role as a way to enhance brain's math faculty.
「珠算の将来」　上野　健爾　京都大学名誉教授