First of all, while I’m far from an expert, a cursory bit of Googling indicates that you are likely wrong about homosexuality being “always a shame in Russia.” It appears that homosexuality was openly accepted in Russia until the Europeanizaton of the 17th century.
But that is beside the point because Rashemen, while being inspired by Russia, is not Russia, so WotC can give that society whatever attitudes they want towards homosexuality.
And that still is beside the point because even cultures that condemn homosexuality still have gay people, so nothing in the Rashemen cultural attitude precludes Minsc being gay.
I’m pretty sure that most people in Russia don’t care one iota about a character from a fantasy land loosely based on Russsia in a video game being optionally gay.
