Casshern is as heterosexual as you can get while being so asexual that sex and sexual desire are completely foreign and confusing concepts. He doesn't have fantasies or dreams that are more memories of his past and he's programmed to be heterosexual, so doing anything with a guy of his own initiative is never going to happen. Not to say it's completely impossible though.
Dib is straight and hasn't had dreams of guys and wouldn't know what to do if he did. And trying out a relationship? PFFFT. Flo on the otherhand is a girl who doesn't mind to experiment! Especially if the girl doesn't mind her insurance kink.
Prussia is...a nation and sexuality is kind of twisted with them. He's felt up male nations before, and yeah he's dreamt about invading and seizing their vital regions, and if sex was gonna happen, it would be awesome no matter if it was with a girl or a guy because he's Prussia. He's too awesome to cut off half of his options.
Kana is a little too young to be thinking of anything like and she wouldn't be interested in a relationship with a girl anyway. Rayfell is more an equal player than Johnny, but she still prefers the ladies.~ And she has several fantasies involving what she'll be doing to her cute little Cheryl once she's a full vampire.
And Toboe is...a wolf. Homosexuality isn't that big a deal, but he was too young to be feeling the need to rut just yet. So no fantasies either.
AND NOW THE QUESTION: Kitty has to adopt one of the kids in Marina. Who would it be?