Why is shemale or tranny consider offensive?

It's all in how you use the word, what your intentions are and how the listener hears the word.

I would not think the term Tranny or Shemale would be considered offensive in the Adult entertainment industry. If you call someone that that is a regular girl or guy and not a he/she type, then yes, it may be offensive.

"tranny" and "shemale" are used frequently in both the porn industry, among sex-workers, and even drag performers. Why? The reasons are varied. One is that many of these people have little respect for themselves, so they don't consider referring to each other by pejoratives to be degrading. Some people are just naive, and don't know the origins or meanings of the words. The "pre-op transexuals" (note the different spelling, one 's' vs "ss") you know are not Transsexual women in any medical sense of the word. Transsexualism is a medical condition, and women so affected Transition to correct their bodies to match their minds because they know their gender identity is female. Transition typically culminates in genital surgery. The people you know are gay males who have feminized their bodies so they can have sex with other males for money. They do not want genital surgery because if they lost their penises, they would no longer be able to perform in their profession. *IF* they are suffering from Transsexualism (which I doubt), then they've managed to find a way to do what the vast majority of Transsexual women cannot do - live with a penis between their legs. So, if you refer to a woman suffering from Transsexualism as a "tranny" or worse, a "shemale", you are essentially defining her as a gay male who seeks partial feminization in order to have sex with other males for money, and not a woman who was born with an incorrectly gendered anatomy. Personally, I can't think of too many things I'd find more offensive.

"Though they are in adult film, does that make a difference" YES.