heh, Yuuri of course. (And I can see him with girls too if that makes it more relevant.) Anissina and Young!Conrad who I forgot to add to the list under 4th wall even though I probably 4th walled him the most too. Actually just everyone I've ever played from KKM because it's like bisexual world.
I don't think Kitty has dreamed or fantasized about it, but she might be willing to give it a shot. At least sexually. I'm not so sure about an actual relationship. She did not hate making out with Faith during the mistletoe.
Spike has done something in his past. And I'm guessing that was likely a threesome involving Dru and Angel. And he's probably had some dream about it or something similar since that really annoyed him. I don't think of his own accord he'd follow up on an opportunity. Could a girl get him into another girl/guy/guy threesome? Yes. He's love's bitch. A girl can make him do anything. He assures me gay hate sex doesn't happen as often as it does in my fanfic. Whatever, muse, I can dream.
Q appears as male but I think the Q are probably genderless. So, he might appear to court men (which he kind of did with Picard, no?) but it's really no less odd than when he does it with women like Vash or Janeway. The Q he was with in Voyager appeared female for whatever that means. Possibly could have just been because they were taking up human form and assuming the most common roles. He also doesn't sleep so no dreams. And fantasy for a Q is reality. Usually.
Sorry, Nina. That's a no on Euphie. Would she kiss a girl back who was kissing her? Probably. But a kiss is just a kiss~
And as much as the fandom would be sad (enraged? idk) to hear me say this, I think Sam pretty much just likes the girls. Any scenario of Sam and a guy I think of is based pretty solidly on dysfunction. But hey, he is full of dysfunction, so it could happen.
Xander has probably had dreams and possibly fantasies. I think he's perfectly happy to be with girls. I think he likes girls and is attracted to them. I think he prefers them. I also think...maybe it would just take the right scenario for him though. I do think he'd resist it, but he might give in.
I bet Garcia has experimented and maybe even dated or would. And dreamed and fantasized. But I think she'd ultimately be with a guy.
Sokka...nah. (This is partially because despite dating in canon my brain goes "too young" and shuts off.)
Cornelia would be with men. ...Or Euphie. Euphie doesn't count as same sex. She is just Euphie-sexual.