Take your pick from the following reasons.
1) "Everyone" isnt saying that
2) Its the UK way that when someone or something is ultra succesful (as BG3 has) to then knock it whenever possible. For some reason the British just hate success.
3) Because, particularly on the internet, people like to be "edgey"
4) Because the slightest piece of non heterosexual item, even when its as slight as it can be in BG3 if you make certain choices, gets people all worked up
5) Because people like to troll for attention
6) Because it gets more hits to their social media channel
Having said all that, would I recommend that you play it, no, absolutely not. I find that if people go into something, be it movie, tv show, book or game, with a negative slant, they then only seem to repeatedly find and exaggerate the negative. I'd suggest that you already have that negative seed planted in your mind and if you play it, that seed will only grow. So my advice would be that you dont play it.