TBH sex in Mass Effect (and videogames in general) is kind of just a selling gimmick, so it doesn't matter too much. I fucked Miranda, Jack, AND Tali, the only one of which I actually made a point to fuck being Tali, just for the sheer weirdness factor, but the fact that I was able to nail Jack and Miranda without even trying says something about the design of how the relationships work (Jack is female BTW for those who haven't played the game.)
Ok, so now, rewinding back to my first point, sex is a selling point, and judging by the amount of times I hear "******" as a derogatory on xbox live, the main demographic is strait males, here it's catering more to strait males who jerk it to videogames, so pretty much, horn dogs who don't know how to interact with real women. So you have to offer strait sex obviously to sell and it to those who want a game with porn attatched, but to hook those who JUST want to buy the game for the sex, lesbian sex is pretty much a definite sale.
Now, to fit these two together, your main demographic here is strait males who may or may not be homophobic. Getting Jack and Miranda to jump on Shephard's man-muscle pretty much just amounted to being nice to them. So hypothetically if you could do other guys, it would probably work about the same way. Assuming the player doesn't intentionally act like a total dick to everyone, the situation would arise where friendly conversation with Thane would turn into homoerotic sexual advances, which would be a bit offsetting if you weren't gay and/or trying to bone them.
Also, can you romance all of the female options as female Shephard, or just Jack? Because Jack makes it known that she exercises sex liberally and doesn't really care about gender. None of the male characters make any indications to being gay, so it might just be for consistency's sake.