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.
Don't ask Google. I'm Russian and I know the history, mindset and traditions of my country well enough. Let's stop at that, please. Any Russian still living in the country can support my claim.
As well as the fact that a lot of Russian gamers I personally know do count Rashemi as a somewhat-related people.
And we aren't talking about most people in Russia, just those who are interested already/could be interested. By no means I'm saying I know it all, but I would not have said anything about that inconsistency if I wasn't sure.
WotC and Hasbro can do whatever they want. As did V5 VtM. Look how it's turned about after mentioning Checnhya:
https://www.polygon.com/2018/11/16/18098929/white-wolf-controversy-paradox-interactive-new-ceoIt would be not a nice thing to offend people and their culture, when you can easily be more diplomatic toward the whole case and people's beliefs and feelings.
