Well, the moral values in Japan aren't based on the bible or the church. They are something among people and are often not explicitly told. Japanese people hate holier than thee attitude more than anything so they won't preach you, but that doesn't mean there is NO moral obligations. Japan actually has a lower abortion rate than UK.
http://www2.ttcn.ne.jp/honkawa/2247.html
The data can vary depending on the source you refer to, but I doubt any of them show you a considerably higher rate in Japan.
Girls don't talk about abortion in Japan either. I have yet to meet a girl who confessed to having an abortion. Single mothers are relatively not common in Japan, so adding that to the statics, I can only assume they are more careful or they get married if they do get pregnant.
Anyway, I'm not sure about the era right after WW2, but nowadays, most Japanese girls who marry American military guys either go to the US, or follow their husbands around the world. There is a slang, ミリ妻, to describe these wives, and while I assume there are many problems due to cultural and linguistic differences, I don't think marriages between US military personnel and ミリ妻s are something that only last until the husbands go back to the states.