I think you can support and sympathise with individuals without supporting the trans concept.
I have two good friends, both MtoF trans (and are a couple). I love them and supported them through their 'coming out' to friends and family and one through her operation.
Though I have never gave them an inkling of how I feel, I don't truly 'get it'. In order to receive hormone therapy and the operation, she had to live as a woman for a certain period. This involved wearing dresses, make up and jewellry. They carried on in the same jobs, same hobbies, just changed their outward appearance.
This does make them happy, which I am, of course very glad about, but it doesn't make them women. It makes them men wearing dresses. Being a woman involves having female sex organs.
They have no true concept of what it is actually like to be a woman, just an idea of how they imagine it to be (as they are both physically very large and unlikely to ever 'pass' they will never truly know what it feels like). The reality is feeling vulnerable, wary of assault and abuse from men, criticised/praised for looks or lack thereof, pressure to act a certain way, discrimination at work, lower pay, menstruation etc.
I can't imagine anyone actually feeling they want that ^. They don't know about that. That, coupled with a vagina and uterus is being a woman.
I dislike my breasts (they are difficult to support, are often painful and get me unwanted attention since the age of 10), hate my heavy and painful periods, avoid make up and skirts, have a manual job, dislike sewing, would love to have a shaved head for ease of management and quite fancy having a penis. Am a man? No I am an individual. Are my friends women? IMHO, no, they are men who like to appear as stereotypical women.
It's the fault of our disgustingly gender-sterotyped society that decides 'men do and look like this' and 'women do and look like this' that makes individuals who's interests do not match with their genitals feel so out of place and upset that they are convinced they are in the wrong body. A man who likes make up and dresses is made to feel so absurd and perverted that his only option is conclude he is, in fact, a woman.
Having said all that, society is how it is and while change is happening it's too slow for trans people who are suffering now. So, yes, men and women should be able to dress and act how they want without persecution, but at the moment they can't, they have to justify their choices and behaviour by attempting to become the opposite sex. Of course, often they never really achieve this (due to physical size/characteristics) and are still discriminated and persecuted. I've seen the abuse first hand and it's sickening.
What's the answer? Total overhaul of society, destruction of stereotypes, gender roles, prejudice and discimination so people can act and behave however they want...Simple! 