I have to wonder exactly what fantasy it was ruining for those fangirls. Maybe they were really into the idea that Kaidan was a hardcore breast fanatic or something? 
I don't remember... I just read a couple of such posts in 2012. Not sure they even stated what exactly pissed them off about it. It's not like Kaidan came out as gay and dumped femShep. Now THAT would have pissed *me* off, being invested in the romance, being told how much they are in love and them, oops, he's actually gay. That would have made no sense to me. And I don't think Bioware is ever going to do something like that - ever. Not the kind of realism anybody wants, I think.
But finding out that your love interest is actually bisexual midway through the game? Why not? Doesn't change the significance of the relationship your character has with them. Dumping your character for the opposite sex, however, would be a shitty feature. Who wants to be dumped in a videogame (for whatever reason)? Jacob proved how very much that sucked...
One thing that I would find interesting as mentioned above is a love interest who is introduced with a certain sexual orientation but then slowly falls in love with your character and realizes that they are actually bisexual or even gay. So said squadmate would initially not be a love interest and have neutral conversations.
I kind of roleplayed that as an alternative ME3 scenario for my canon femShep. She was with Kaidan but then Liara turned out to be so much more supportive. And she kind of stole my heart in ME3... so I was VERY tempted to turn that wonderful friendship into a slow-burn romance. I think it's a beautiful setup. Having been friends for two games and then realizing that you are actually the perfect match (doesn't have to be about being gay/bisexual). My femShep was straight, but I almost made her bisexual for Liara (or asari-sexual, still not sure if being with them counts as lesbian/bisexual).
I doubt ME:A will be another trilogy, so slow romances that take longer to build up won't happen. But I'd love that. Having to earn somebody's trust and friendship through certain story-related events. Imagine you'd have had to first save Ash/Kaidan on Virmire, build a friendship for the rest of the game, then trust issues in ME2, and THEN in ME3 they are so relieved you're still one of the good guys and you celebrate the reunion with an (unexpected) kiss. That could play out as a oh-I'm-bisexual romance too. Which I guess this is what they tried to do with Kaidan? I haven't played a dudeShep romance with him yet. So I have no idea how it played out...
However, romantic drama, especially when it's about self-discovery or coming out, gets cringe-worthy easily. So if we were to get a surprise bisexual romance, I'd rather it not be too big a deal. Make it a sweet but short scene.