Hey Ina,
I think the answer may be two part. first being biological, the Second being sociological.
The biological i'm not going to mention in depth, but i think you know what i mean, after all we are talking about sexual relationships here.
In terms of the sociological. Men still tend to take the lead, When a Westerner has a relationship with a Japanese, they tend to live in the west.
Because a western country will be more accepting and open to foreigners than Japan.
Regarding Mixed Race children especially, you would want your children living in the west.
Lest they get bullied and called Halfu
But yea, In general, let me just repeat that.
In general. the woman tends to be submissive to the man in a relationship.
That is not to say that the woman doesn't make decisions or have power.
Most relationships tend to be 50-50, yet when it comes to a tie breaker. The man will put his foot down, or let the woman have her way.
This combines with:
The White woman will be having to deal with racism if in Japan
The man may feel secondary to the woman if they live in the west, thus is unlikely to want to settle in the west.
Also by tradition, he would forfeit any inheritance due by doing so (again, in tradition).
Also, Japan and Asia in general tend to have the relationship model that the west had in the 50's.
And while many support this model, and think it is a good thing, It goes against feminist ideals, which have become standard in western culture.
In Japan a woman is expected to do all the cooking, all the cleaning, etc.
So if you are a western woman, These will be duties that you are expecting to share equally.
Not so if you are with a Japanese man.
Again, In a relationship the man on the whole has the tie breaker.
If the man expects his "wife" to do all the cooking and cleaning, then he will not get on well with a woman who expects him to cook and clean as much as her.
This combine with the biological aspect, make Japanese men an unatractive prospect as husband for the modern liberated European woman.