It would be up to that person to say and yes I would like to know. If a person just does not 'look' the part, and given the increase in frequency of this occurrence of this change, a conversation along those lines would suffice. I believe most persons would state so if indeed they don't 'look' the part. However for those that DO look the part, they may decide to not tell.
Would I still be attracted if I knew? That's a good question, I'd take it by a case by case basis. For instance, let's say that a person is not into gang-bangs and orgies but rather one-on-ones and threesomes and he/she meets someone they are very interested in. After a great time of finding out more about each other and of course flirting that person then comments on the great weekend he/she had in this orgy/gang-bang the other day. Even though there's a first attraction, additional information can change it and depending on the level (or intensity) of attraction he/she may decide he/she is still game.
Thus it may be a case by case basis. However for another person it may be a situation where once you were not born that way its NO WAY.
Nice question though - kudos