I bet that even among foreign fans, there are more males than females. It's just that most of the males lurk, because there's a stigma in being a fan of this stuff. I became accustomed to it in the J-pop fan community, but I'm seeing it all over again in the metal fan community, thanks to BABYMETAL. There are only so many times that you can be called a "Jap-lover" or a sissy or a pedophile or a creep before you give up and go back into the shadows.
Being a girl doesn't necessarily make liking jpop more acceptable. My mom asked if I was gay at 15 because I watched Hello project mv so much. I'm not and I was confused why she would think that. It's just seen as weird to people who don't get it.