Now I don't know Witcher 3's handling cause I have never played it, but I have surely heard more good about DAI's handling than Witchers. Also doesn't Witcher like have just one gay character in the all of the games or am I mistaken?
Mislav, the hunter
A gender fluid Elf clothing merchant
Ciri is a Lesbian or bi based on player choices
Dethmold, one of the main characters in TW2
Philippa Eilhart (in TW 2 and TW 3)
There could be more but IDK.
As for the different handling between TW3 and DAI. I don't think it would be fair to compare Dorian and Mislav. I think comparing Mislav or the elf merchant and Krem is more fair because their roles are minor. With Krem, I felt that BW stood on their soapbox for that and it got kinda cringey.
Krem mentions his boobs in jest, gets angry if the player doesn't automatically assumes he's trans, IB proceeds to give long winded lecture. In this case the player gets talked at rather than talked to.
With Mislav and the Elf, it's an interaction, a conversation. With Mislav Geralt asks him why he's out in the woods alone, Mislav hand waves it at first unless Geralt presses it (Geralt mistakenly believes he's a werewolf and offers to help). Mislav tells his story, Geralt can leave it at that or says he's sorry for what happened to him. The Elf Merchant can be found while on a main quest, he's mistaken for one of Dandelion's lovers. Geralt was searching for a woman and the Elf puts on his dress to show him that he's the woman Geralt was looking for. Geralt can ask why he dresses up as a woman, he simply gives his reply that he dresses as both, why he does it, and how it makes him feel. I think Geralt can remark that he shouldn't have asked, but either way it's done.
Dorian was a well written character which was letdown by a humdrum character quest.
This sums up my feelings about Dorian perfectly.