Except it adds exactly zero replay value for people who don't want to, say, play as a purple haired male dwarf with the urchin background just because that's the only way to play a certain romance. That limits the player immensely in their choices and that's just about the opposite of what you want in an RPG. The only thing I think is reasonable is if it's based on alignment (and afair, there have been several games like that). But everything else just sucks, yes, even gender locks.
And seriously, nobody is stopping you from headcanoning that a character you're currently not romancing is not interested because of your character's gender, race or alignment. Most games are not like BG3 where just talking to people three times will make them want to aggressively jump your bones.