How would you describe the meaning of 弯 wan1? Specifically regarding its use to mean someone who is 'bent'? Someone who likes someone of the same-sex or is at least suspected of it? Does it include bisexuality? Is it polite or rude, subtle or overt? And when did you first start seeing this word?
gay = not straight = 不直 = 弯

I first heard this word a few years ago. It is often used by younger people.  It is a slang and should not be used in any formal situation. Like any slang, it can be rude or not depending on the context.

I do not believe it has any sexual reference though.