It's that Muslims should marry Muslims only. But consider Iranians are born Muslims whether they like it or not, they will be penalized by death if they want to convert from Islam.
There is something in Islam which u can temporarily marry someone. For this men can be with non-muslim women (have to be monotheist). But for marriage even men are not allowed to stay with non-muslims.
You can change to Islam just for the sake of formalities and then live your life.
I think if you marry a girl and she renounces her former citizenship, there's no need to have a marriage back in her country. Simply you're two Japanese living together.
Majority of Iranians love to marry a foreigner and live far away from their country. They don't like religious repression. I think everyone should leave Iran until no one is left!