To lighten up the mood a bit on this cirious topic, allow me to make a little joke.
Because I'm laughing tears and I just have to post this.
Oh my God. Masterpiece.
So...hey, you guys already said a lot, including my opinion, so I should not reply... But, here, my two cents.
Geralt is eterosexual, that's why. Some of his personality traits are already defined. The game and the lore in general have nothing against being homosexual indeed. Great characters as Philippa are homosexual for example.
Also, sexuality is a topic well covered in the book saga, and gently.
If then the world they live in can be sexist ... well, come on, it's like in real life, there are those who are sexist and who are not, as in reality.
But let's not talk about sexism because there's already a topic about this.
I understand that this may be a limitation on the freedom of character building, but this should not undermine the appreciation of a game or condemn it. Moreover, the discourse on sexuality is based on a broader view of things, then it is good that this view also applies in accepting certain characteristics of a character, as his heterosexuality, bisexuality, or homosexuality. Geralt is heterosexual, period. Philippa is homosexual, period. Other characters are bisexual, again, period.
A player should appreciate every single aspect of a game well done with some sensitive issues dealt in a mature way, in fact, in a gentle way. I know the point of the matter is the fact that this is a role-playing game... but here the protagonist has traits already defined. it is wrong that some potential players go away because they do not find themselves in the character's sexuality. Among the other 90% of the sex scenes are absolutely skipable. Apart from this... you are free, you can shape the story, you can modify your role in the plot of the game, you have influence.
If the player manages to define the sexuality of a character and then his choices have no influence in history? In the end what is left? "Wow, I've created a homosexual character, this game is fantastic, so everything should be like this in RPGs!" The story of The Witcher is not based on how many heterosexual relationships one has.
So even if Geralt has some built-in features you should try to enjoy the game, you'll see how each character is exquisitely characterized, both positively and negatively, how this does not undermine the care and the passion with which the game was developed, with love for the lore.
Oh and welcome to the forum.