It's adorable how the writer gave the gary stu a magical power that makes him an irresistible womanizer who's sterile and immune to all STDs so he can screw everything that moves without a care in the world. Now that's how you write fantasy for your intended audience.
In the Witcher games you yourself can choose not to **** anything in sight if you dont want that.
And over the course of 8 books that he has only had sex with around 10 named characters.
As a Witcher he was subjected to mutations and because of his high tolerance for them he was subjected to more than usual and as a result not only became sterile he lost all his pigment aswell.
The purpose of the mutations is not only to make them faster and stronger but also resist deadly poisons.
Poisons from various monsters and and potions that enhance their witcher abilites but are so toxic an ordinary person would die from it.
Why the **** would he not be immune to an STD then?
Sorceress are also sterile (there are exceptions) it's just a matter of gaining something in this case power and losing something in this case fertility
Yennefer desperately wanted a child and she couldnt achieve that (It worked out because of Ciri though) it's not just a matter of "well i can **** anything that moves without consequence hahahahahah" theres something tragic about it aswell.