For me, the best part of games that have a large amount of agency and player choice is they often have characters who are effectively their own people.
Characters you will develop a relationship with based on your characters actions and who they are, rather than created to hang off the player at their whims.
I hold Dragon Age Origins and Mass Effect 1 as exemplary examples of that. There are characters in those games that amazing relationships can be formed, or those relationships can turn bitter and have them betray you (although technically it could be argued in some cases you betray them first).
When it comes to relationships, if you make a character playersexual, there solely for the romance of the player however they desire, you take away the weight of them being their own character. A character with their own desires, motivations, and story.
You won't get to experience your character engage in a way unique to them. You won't get to go through different playthroughs experiencing different relationships. You won't get to come onto forums like these and engage with others about how they built relationships.
Making characters playersexual cheapens and weakens the whole structure of meaningful roleplaying, where the point is for you, the player, to roleplay different characters to experience different arcs and stories.