I never tried and I'm not too sure, but I have a feeling that Shep can only hook up with the human of the opposite gender. May be wrong about that though (went for Liara both times).
You're not wrong.
StraightShepard is a mini controversy... and while I appreciate that they stuck with making people straight in the universe (as opposed to the ambivalence of Dragon Age where sexuality almost becomes a joke), I suppose I can see why some people are pissed.
Not that I wouldn't want Gay/LesbianShepard, but this is a game where FemShep is a billion times better than MaleShep but BioWare INSISTS on using bald space marine as the cover model because 17 year old boys can't handle a girl with guns who isn't just a walking pair of breasts. Oh well.