same sex acceptance is not because FR shares the same moral values as our modern world.
it is because Greenwood wanted everyone to be bisexual due to him imagining FR as a world of sexual debauchery.
Heres a pointer for you. I FR, incest is considerd a normal family pasttime. Hows that for modern western morals. and no, pornhub is not a standard reference guide for modern western morals.
Yet, 20 years ago, same-sex romance in BG2 was unheard, I do not even recall, if there was a single gay NPC in that game, let alone the notion that "everyone was bisexual". Drow were only starting to become accepted, Tieflings apparently not so much. And now in the year 2020 we had goblin companions, transgender companions, we can make drow and tiefling characters and general of almost every phenotype and skintone. We can also bonk anyone and there might even be nipples. And that holds true for many, many games of the last ten years, be it Mass Effect, Dragon Age, PoE even the Fallout series (which naturally started out more liberal).
Political issues like segregation, suppression, dislocation, and especially racism plays a much larger role in the recent years, sometimes even central. As the whole society became more liberal and accepting (on these issues at least), so did it's media.
On the topic of incest, well I suppose then genetics work differently in FR, because there are existential drawbacks of bonking your family. Additionally you do not have to go back far, like 50 years, to see that marriages within the family were not unheard nor frowned upon and sometimes even the norm.