Why is shemale a offensive term to a pre-op male to female transsexual?

The only trans person (male to female hasn't had gender reassignment surgery) I know works in the porn industry and I learnt the term from her. I've only known her for a few months. I thought it was the correct term to use, and just referred to the fact that full surgery hasn't occurred yet leading to a... show more The only trans person (male to female hasn't had gender reassignment surgery) I know works in the porn industry and I learnt the term from her. I've only known her for a few months. I thought it was the correct term to use, and just referred to the fact that full surgery hasn't occurred yet leading to a male to female with both male/female anatomy. I recently used it when I met another trans person (also in the porn industry) and she told me it was considered rude, she said it kindly it wasn't an argument. I stopped using it but I was to embarrassed to ask her why it was offensive and what the correct term is. I usually call my friend a women as that's what I consider her.

I just would like to know why it's offensive and what the correct word to use when referring to a transsexual person. I'm a little new to what's acceptable and what's not, and I seem to keep putting my foot in my mouth :{
Update: @ Michele I'm gay so no you wouldn't be the subject of my desire or any women for that matter. Maybe don't be so quick to pass judgement.
Update 2: @ Clones & Michelle I wish I had never asked now. I asked so I wouldn't cause transsexuals offence, I was trying to learn. Only to be told I objectify women (misunderstood you my back toe 'I'm not your object' that has no other meaning but to refer to me) and get angry responses. What was the point... show more @ Clones & Michelle I wish I had never asked now. I asked so I wouldn't cause transsexuals offence, I was trying to learn. Only to be told I objectify women (misunderstood you my back toe 'I'm not your object' that has no other meaning but to refer to me) and get angry responses. What was the point if your going to get angry at me for trying to understand. I never said I get to decide, and I stated in my question I stopped using it the minute I was told. Shouldn't of bothered asking.
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