Wasn't there some ridiculous PR spaghetti-spilling back then saying that Shepard wasn't gay, and Liara "didn't count" cause she was an alien ?
Yeah, that's what this whole ridiculous line of reasoning stems from. It came from one of the devs (Casey Hudson, maybe) responding to a critique about the lack of m/m options with "Shepard isn't gay". When it was pointed out that Fem Shepard certainly could be gay, he responded with that 'monogendered means that she's not actually a female' nonsense (despite the in-game codex calling asari "an all female race") so she's not gay either.
Then, when ME2 came out and Kelly Chambers was introduced, they couldn't use that argument any more and suddenly in the next game male Shep finally got his s/s options too. You'll also note that they most certainly did include Liara as a f/f option in ME3, btw.
Obviously I think it's crap that Shep can be a lesbian but not a gay male. But I would have had way more respect for him had he just said, "You know what? We find sexy blue alien on female PC action to be sexy, so we included it and we don't like m/m action, so we excluded it." I would have been pissed, but it would have had more conviction than the mental gymnastics he was trying (and failing).