On my end, I'll say it's shallow if it's coming from straight male players because it's not about roleplaying FemShep, genuinely considering her feelings as a woman (which guys will never get, they don't have the brain chemistry, just like I'll never be able to climb inside of and fully understand the mind of a male =p), and responding accordingly. It's just about them getting their rocks off with omgzhawtlesbianz!!
That's a bit, as we say in my language, "too short through the curve." Not every male player wants romances for the OMGlesbosecks! I'm a male player, and I was disappointed that Female Shepard/Tali wasn't a romance option, because my character built a strong bond with her in the original Mass Effect.
I'm not really annoyed at Bioware for not making Tali (or another character) bisexual. But from the looks of things (I haven't reached the end of the game yet, so I could be wrong, mind!), there won't even be the possibility for a female Shepard to even express any possible feelings towards her. I'm a role player first and foremost, and the possibility for my character to at least *be* in love and *act* that way not being present, that's feeble. I'd settle for unrequited love, for instance, your love interest simply rejecting your character but still developing a strong, romance-equivalent bond would be more than enough for me.
It's not the fact that the characters aren't bisexual, it's the fact that the player simply doesn't have the option of even attempting a romance, even though it will end in failure, because the other character simply doesn't swing that way. I'll take Tali as an example. For instance, in my game right now, all that's happened is Tali's off-hand remark that she'd feel honoured to link suits some day. If a female character could have had the option, at that point, to express feelings toward her, with Tali rejecting her, but still assuring her that they have a special bond, then it would have been all good. Miranda could have had a similar dialogue exchange (and I understand Jack actually has a short exchange with Shepard concerning her heterosexuality, but even then, it's brief and without actual consequence).
I had much preferred a Shepard with a broken heart, and have the game acknowledge this fact and incorporate it in the story, for instance by adding Tali's photo to Shepard's cabin, and having a broken-hearted Shepard stare at it in the end, like with the loyal-to-the-ME1-romance ending, only with a sad undertone instead of a positive one.
Hell, I'd even preferred it over an actual romance.