For better or for worse, I'd also shy away from someone who has a mouthful of sharp teeth bigger than my head. Bros forever, though.
Wait, how would the Krogan work though.....O___O
Oh you guys! Use your imagination! I mean, asari have sex telepathically, right? And turians have boney plating. And quarians are contained within suits but can just take a pill and make it all better. It's the beauty of sci fi. Again, I know it won't happen, but I think it would be great.
To be honest, I really just want to see an unapologetically stereotypical super butch lesbian and I think a krogran could be a great way to do that. I could just imagine the PC asking her about the genophage and her sexuality and her literally just laughing in his/her face and not dignifying it with a response. I think Bioware is a bit intimidated to create a stereotypical butch lesbian because she'll be seen as a stereotype solely, but I think it would be great. Like a Lea DeLaria esque butch gal.
My problem isn't that Bethany is sane (my favorite ME characters are Kaidan, Miranda, and Kasumi, so I usually favor the more subtle ones over the super, over the top, angsty, look at how bad*** I am ones), it's that she has zero character development if you don't make her a Grey Warden and is essentially just a plot device in order to make non-mage Hawke care about mages rather than a genuine character in her own right. And then if you do make her a Warden, you have to live with knowning she's completely miserable. It has to do with poor writing rather than her personality and it's compounded by the fact she's mutually exclusive with Carver who is an excellent character even if he's not likeable.
Agreed. Bethany was suuuuuuuper bland. At least Carver was interesting. He was annoying, but interesting and had great character growth over the game. Bethany started blandly vanilla and sweet and ended that exact same way. She just never stood out for good or bad, to be honest.
Functionally, in-game, they aren't really bisexual. They're protagonist-sexual. They can go either way, but they don't really go both ways.
Kaidan? I played as a male Shepard, and never made a pass at him; I never caught a whiff that he was interested in Shepard the whole time.
Kelly was designed to be romanced; if she wasn't, she wouldn't be romanceable. 
Similarly, with Liara, she'll express an interest in Shepard, whatever Shepard's sex is, but I don't recall Liara ever expressing that she was interested in the sex opposite to Shepard's. We know from the games that the Asari are generally not at all picky about the gender of their partners since Asari themselves are mono-gendered, but it's not out of the realm of possibility that certain individuals have preferences or aren't interested in male or female aliens.
Compare Zevran and Leliana in DA:O, who were both clearly and openly bisexual. Even if you don't romance him, Zevran will mention that he's been with men and women. Leliana will express interest in you if you're a man but will also talk about her relationship with Marjolaine, and provides a pretty good outline of the traits she's attracted to. All-in-all, Leliana was a good romance; I remember reading about her being pretty popular, and I think it's easy to see why. She had better writing than the comparable ME romances, and better characterization too. I think the way in which her sexuality was presented is a part of that.
That's my point: if making characters bisexual, I think it'd be a lot better to make them actually bisexual, not just attracted to the sex of the main character.
There's alot to talk about in this post.
First, Kaidan flat out flirts with you when he's in the hospital bed. And, if I recall correctly, it happens without your initiating it which was a cause for complaints from many a straight guy when it came out. He says, "<sexy chuckle> Are you flirting with me Shepard?"
Second, a bisexual person doesn't have to immediately express interest in both genders or have their bisexuality card revoked. Fenris can be romanced by men and women. And he'll sleep with Isabela if not romanced. Anders can be romanced by men and women. In either case, you can hear banter about how he's slept with Isabela, he expresses interest in women in DA: A, and, if you are male in DA2, he'll discuss his past with Karl. I mean, those are examples of bisexual men.
Not every bisexual man has to say: "I have been with both men and women in the past" (a la Zevran, who, coincidentally, I think is the worst of the bisexual men because he was clearly written with a female audience in mind and the m/m stuff is added on and hidden behind multiple "Is it okay if I flirt with you? Really? Because if you say yes, gay stuff will happen? You're sure that's cool?" dialogue options). The only qualification for being bisexual is being able to be romanced by both a man and woman. This concept of 'playersexuality' only exists in the fact that all LI's will sleep with a PC regardless of their looks, attitudes, and (for the most part) behaviors. That's playersexuality. Not a bisexual character.