[GoNative]

I was married before coming to Japan so I have no direct experience of dating Japanese girls myself but I have plenty of single friends. One of the things I found a bit different here is that it seems to be considered boyfriend and girlfriend one has to ask "will you be my boyfriend/girlfriend?". Until this question has been asked and accepted it's not really considered they are together and it would be acceptable for them to see other partners. They may well be having sex but without actually asking the question it's not really considered a close, monogamous relationship.
In Australia we really wouldn't actually ask that question anymore. It would generally be considered a little juvenile, school yard sort of thing. If you're dating and sleeping with someone it's just a given that you are together without explicity having to ask such a question.