How old are you now? What is this job that has you so worried about what you did in Japan when you were 18?
I don't know what the laws in Japan and the US were when you were 18.
I guess that, if both sets of parents approved of your dating relationships, and all you and your girlfriends did was consensual, and no police complaints were filed, or arrests or charges or convictions resulted that would show on your criminal record, this is a nonissue.
Consult with an attorney in your area if you find yourself unable to concentrate on anything but your time with the girls in Japan.