2 things here. First, the law does not prohibit surrogate births, BUT lawmakers want to impose a ban on it and penalize those who violate it.
Secondly, it is the doctors themselves who restrict in-vitro insemination to married couples
for ethic reasons. What kind of ethic is that ? I knew that Japanese were bad when it came to philosophical or moral reflection (ex. you can't drink even a beer legally before 20, but anybody can go to love hotels as teenagers). I can't understand why Japanese insist so much on couples being married to recognise children. In Western countries, children of unmarried couples have been recognised since the DNA tests became accesible a few decades ago. In Japan, even with a DNA test proving who is the father, if the parents are not married there is no way to legitimate the father. Very backwards indeed.
But what if a couple uses in-vitro insemination to have a child (without surrogate mother) and are not married ? Japanese may not understand that some people do not wish to get married even though they want to live with one particular person and have children with him/her (that is surely beyond most Japanese's imagination

). I was told, "but why would you like to do that ?", "why living and having chidlren with someone and refusing to get married ?". For lots of Europeans, the reason is obvious. "marriage" is associated with religion, and they do not want to be involved in religious ceremonies or traditions. Furthermore, marriage is troublesome, especialy in case of divorce, and people do no want to have to deal with slow tribunals and expensive lawyers when they could settle things by themselves. I guess there are other personal reasons not to want to get married.
Then,
But don't Japanese laws accept surrogate births ? (see first qutote) I thought it was Japanese doctors who were opposed, which would explain why the couple went to the USA to do it. The problem here is that the Japanese government does not recognise "adoption" at all. I knew that adoptions were not common in Japan, but I didn't know they were illegal, especially when adopting your own genes !