It seems to be a show-only thing. After all, there have been a number of very prominent LGBT figures in the history of Westeros – Laenor Velaryon, Prince Daeron, Daemon II, etc. – and you’d think that if it was as significant a sin as it is on the show, it would have come up when these figures were potential candidates for the Iron Throne.
However, there doesn’t seem to be widespread social acceptance either, given that (for example) the majority of the LGBT characters we encounter get married to people not of their sexual orientation, although this seems as much (if not more) about pulling their weight in their family by producing heirs and making dynastic alliances as it does about remaining closeted.
Indeed, if we can take Cersei’s ruminations about Renly as any metric, rather than seeing sexuality as something set in stone, it seems to be described as a general preference but not an exclusive one: “a man may prefer the taste of hippocras, yet if you set a tankard of ale before him, he will quaff it quick enough.”
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