It's not 'merely' the LGBT community, I'm a guy in real life & I play female characters, yet I also like to date female characters. But this is a bit besides te point.
The reason people probably assumed that LGBT relationships would be available is that it wasn't mentioned anywhere that it'd be straight relationships only & that Bioware has a history of adding at least some kind of LGBT content to it's games (see list below).
Kotor had Juhani
Jade Empire had Silk Fox & Sky
Dragon Age I had Leliana & Zevran
Dragon Age II had Leliana, Zevran, Isabela, Merrill, Anders and Fenris
Mass Effect had Liara
Mass effect II had Kelly Chambers, Samara and Morinth
And last but not least
Mass effect III had Stever Cortez, Kaidan Alenko, Kelly Chambers, Liara T'Soni and Diana Allers
A lot of these characters were available for both genders and some of them only for the same gender.
Let's say I own a pie restaurant & you happen to like apple pie. You reserve a table at my restaurant and order an apple pie, after which i tell you that unless you visit my restaurant on a weekly basis every single week for at least a year and reserve that special table in the corner, I'll only serve you chocolate pie which, if you're willing to work with me here, you don't like.
I wouldn't be surprised if you felt discriminated against, whether that feeling is correct/justified or not.
This is pure capitalism. Nothing more & nothing less. Since we live in a capitalistic world, this is to be expected. It's also why I won't bother complaining about it (I can't change the world).
To use my pie restaurant as an example again...
If I wanted to serve apple pie to customers, I'd probably have to hire a new cook or at least a cookbook that contains the recipe. I'd have to adjust my kitchen, so that the apple pie dough wouldn't be contaminated with chocolate, etc. (give me a break, I'm making this up while writing ).
Of course it would have been smarter had I initially reserved a part of my kitchen for apple pies, but alas, I wasn't that smart. Still, I need to make a profit (I need to eat & pay my rent as well (once again, we live in a capitalistic world & i can't tell the land owner that he'll have to forget about the rent for a few months just because I made a mistake)) so yes, I'd charge the people who want this. Not too much, as it'd scare them off, but still...