The realistic argument falls to the wayside in a game, and a game set where dragons swoop from the sky and a giant hole in the sky connects to a world of demons and physical dreams nonetheless. What you mean to say is "believable", and I'm willing to sacrifice believability in favour of player choice and greater access to narrative plot threads.
Besides, if I can believe a man can be corrupted by a disease to the point of ascending godhood and commanding a dragon made of red lyrium, and if I can believe that magical beings walk the Earth, and if I can believe that green gates spit out demons and can only be sealed by a mark on someone's hand, I'm willing to believe 4 bisexuals can exist in the same space at once.
In regards to sexuality being an integral part of their characterisations, I disagree. I've never felt like the colour of my skin affected my personality, nor my physical sex, and not who I'm attracted to. They all felt like things I was just coincidentally assigned to me, and sure those labels are descriptive, but in a very superficial way. I understand that my race/sex/sexuality would shape how others would respond to me, but if I were suddenly turned straight one day, all that would change is that I now would seek out males to start a relationship with. If I woke up and found I was West-Asian instead of Slavic, I wouldn't immediately lose my passion for ancient history, or my love of fantasy literature. Both of these switches would change absolutely nothing else about me other than which genitals appeal to me the most and what pigment my skin was.
I'd say unless BioWare is willing to explore legitimate conflict and development in character sexuality - Dorian's personal quest aside - gender-locked romances in games will feel like nothing more than an arbitrary restriction. Sera's existence as a lesbian city elf is never explored. Josephine's existence as a bisexual is never explored, even if she abandons an engagement to a respectable family, and even when it's in favour of a female mage Qunari. Cassandra's and Blackwall's and Solas' existences as heterosexuals is never explored. None of these sexualities shape anything about the characters - they only serve as gameplay roadblocks.
Sera's flirtatious sexual nature isn't a result of her being a lesbian. Solas' introverted and scholarly personality isn't a result of him being heterosexual. Cassandra's romantic ideals and her righteous drive isn't strictly contained within the realm of heterosexuality, and nor is Cullen's desire to better himself and protect others. You lose nothing about these characters if you open them up to all Inquisitors. Josephine's powerful work ethic and considerate personality isn't due to her being written as a bisexual, and neither was Leliana's piousness and belief in helping people in the best way she can.
Dorian and Zevran, for me, are the best arguments for sexuality as characterisation. Though I found Dorian's personal quest lacking for reasons I won't go into here, it was undoubtedly part of what his personal conflict was. Zevran was raised in the Crows and encouraged to be bisexual, so that he could convince a male mark just as easily as a female one. In both of these cases, their sexualities were more than just who they direct genuine flirty comments at.