People keep saying that playersexuality prevents in dept, predefined backstories for the characters and... does it? All the characters already have rich, pre defined backstories. If you mean specifically regarding past romances, were those ever really that much of a thing? At most we only ever heard about one really important romance or we would hear about a string of casual flings. In any case, we get the same treatment here. Gale has a romance that is central to his plot. We never hear about any others because they don't actually matter to the plot at hand, but most relationships won't be relevant. So I really don't think playersexuality gets in the way of that. As for missing out on plots specifically about a character's sexuality, again that assumes we would be getting such a plot but for playersexuality, which isn't going to be the case always or even often. As other people have pointed out, such plots are always given to queer characters and while plots about being queer are worth having at times, they should not at all be the default or even the majority.
I firmly disagree that playersexuality is inherently inferior or lazy. It's no lazier than Larian not changing their movement approach or not including readied action. It was not where they wanted to put their resources into. They gave us this (in my opinion a bit too small) pool of companions, and they probably felt like providing enough of a breadth of romance options would require more companions and time they did not want to devote. Also worth noting is that not everyone is going to want to play a character that can romance every other character in a race/sex locked game. For instance I never play guys in games where I have the option to play as a woman, so that would automatically rule out straight women companions for me. And I can live with that, but it's nicer for me to not have to and see more romances as a result.
It is inferior.
Think about it logically: either you remove the relationship aspect from your backstory or you don't. If you do you have a backstory less rich than if you had it. That's for the lazy approach.
Assume you do keep a backstory containing some reference of past love interests. Then either the playersexual approach falls into that narrative (a win) and then you draw with the case where you had preset interest in the first place. Or the player doesn't fall into that narrative and you essentially made your character bi or pan sexual in effect and now your elaborated backstory is clunky.