What happened here is that the romance concept was changed from something in service of the story to being in the service of the player. Once that happened, the designers were pushed into the long, thankless, expensive, complicated task of trying to give everyone as many options as possible. If the romance exists for the player's gratification, then they will naturally want options that reflect their preferences and tastes.
KOTOR had two romances: One between the male player and Bastila, and another between a female player and Carth. Now check out the options in Mass Effect 3:
Heterosexual Female: Kaiden, Garrus
Homosexual Female: Liara, Kelly Chambers, Diana Allers, Traynor.
Heterosexual Male: Liara, Ashley, Tali, Miranda, Jack, Kelly Chambers, Diana Allers
Homosexual Male: Kaiden, Cortez.
We went from two romances to fifteen. (Although a lot of those overlap, just swapping out one gender for another.) And yet people are still unhappy. They didn't get enough choices. Or the character they like is the wrong orientation. Or someone else got more choices and that doesn't feel fair.
Not only do these romances take place apart from the story, but they limit the writer's ability to design interesting teams. What if the writers want add a character who would be inappropriate as a romance option? Perhaps someone old, or underage, or celibate, or faithfully married, or grotesque. They're already having a difficult time pleasing everyone, and every un-romanceable character makes the job that much harder. This would naturally push the character designs towards a "Fast and the Furious" approach, where everyone on the team is young-looking, sexy, and available. That runs counter to the sci-fi theme of strange aliens and characters from all walks of life.
This also pits the fanbase against each other and breeds animosity. After all, the characters are all made from the same finite game budget, so having one more option for straight ladies means one less option for straight dudes. If lesbians make up 2% of the player base, then it might seem unfair to some people that they get a third of the romance options. But if you're in that 2%, then it probably seems pretty damn unfair that straight guys get so many more options than you do. People on all sides can feel frustrated and to them it feels like BioWare could make it all better with just a slight change in their favor. So BioWare ends up with everyone demanding they do contradictory things, and the only way to solve this is to spend less on the core game and more on these side-romances that only a tiny fraction of the players will see.
While it's easy to blame BioWare for this mess, it's worth noting that they got here simply by giving fans what they asked for. I don't think anyone is in the wrong here. Everyone is just saying what they would personally like from the game, and our preferences are inherently incompatible. A gay man and I can both enjoy another zone where we shoot some more bad guys, but we don't both benefit from the option of romancing Tali. The content in the main story is ostensibly there for the benefit of everyone, but romance options only appeal to a sub-section of the audience, and therefore exclude everyone else.
Personally I'd gladly give up romance options aimed at me if it meant that we could return to the days of having romance plots that meaningfully connected with the main story, but I know I'm in the minority.