The realism argument just doesn't hold weight for me for so many reasons. The limitation of the game mechanic is an important one and one that gets hand-waved away. They have to include at least: 1 straight man, 1 straight woman, 1 bisexual man, 1 bisexual woman, 1 gay man, and 1 lesbian OR 2 straight men, 2 straight women, 2 gay men, and 2 lesbians in order for everyone to get 2 choices in their romance option. Of course it's an artificial restriction, just like the five dozen other artificial restrictions that game developers need to account for. Why does this particular one get so much hate?
I'll give you another example: why are the vast majority of squad members attractive, within Shep's general age range (or their species equivalent of it), and/or single? Some combination of at least two of those? It's super unrealistic to expect that, but it's the case and I rarely see people complaining.
Squad that fit at least 2 of those criteria: Ashley, Kaidan, Liara, Garrus, Wrex, Tali, Miranda, Jacob, Thane, Jack, Samara, Morinth, Kasumi, James
Squad that doesn't: Mordin (not attractive and 'older'), Grunt (not attractive and 'younger'), Zaeed (not attractive and 'older'), Legion (an AI), EDI (an AI)
That's 7 out of 8 humans who are attractive, young, and single. It's totally unrealistic, but no one seems to complain. Why is that? I'm going to wager a bet that it's because they are all available for straight PC's except for one of them. (With the James one-night-stand add in with the Citadel DLC, they are literally all available for straight Sheps with the exception of Kasumi.......)
Not to mention that there's a very simple fact: ALL romances are ALL playersexual, particularly in Mass Effect. All of them. All the romances will be interested in having a relationship with Shepard, regardless of morals, attitudes, actions, or even appearance. Nothing Shepard does, short of not flirting with them (and in a couple of cases, not even that), prevents them from being attracted to Shepard, with the exception of Thane and Tali, who need their loyalty missions completed successfully for Shepard to romance them. Even then, none of Shepard's other actions will have an effect on them willing to have a romance with Shepard.
Even in Dragon Age, where characters do have crisis points, where they'll leave the party if they are dissatisfied enough, will still have these characters willing to have a relationship with the Warden/Hawke/the Inquisitor, regardless of anything that they might or might not have in common. Morrigan will fall in love with a goody-two shoes Warden. DA2 had rivalmances, Cassandra, a potential candidate for Divine, has no problems dating an apostate elf or qunari who doesn't believe in her god.
And then there's the fact that you can make your PC stunningly beautiful or ugly as sin, and it makes no difference. I mean, does aesthetic attraction not play even a small part in willing to have a relationship with someone? The simple fact is, there's not always going to be a reason for these characters to be willing to have a relationship with the PC, but they're still an option. The romances are always playersexual. The romances in DA2 were just all bisexual.