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.
Blame the marketing department. The reason there's practically no marketing stuff with FemShep is to preserve continuity with ME1's marketing after the multi-gender ad campaign for Jade Empire didn't pan out. As for why they didn't go with FemShep in the first place, it might have had something to do with the fact that scifi is still considered to be a guy thing, despite large numbers of female scifi fans existing, but it's more likely that it had something to do with the development process of the game, considering that most of the early ME1 stuff had Ashley's face covered up, possibly indicating that they hadn't gotten decent motion captures of female faces to work with when they made those videos.