Shelflife, what is making you think that people on here 'can't get our heads round' the fact that transpeople exist? Of course we do.
Speaking for myself, what I find upsetting is that there are those who think they have the right to speak on behalf of a disparate group of people as though they are one mass, and who see fit to over-rule parents who have expressly told a 14 year old child that they don't want her to bind her breasts. I don't know what that has to do with not accepting that trans children exist. I know they do, and I support those I come across by accepting them as they present. That does not mean that I believe that they are able to transmogrify into the opposite sex, which is simply not possible.
On the whole, I don't treat people differently based on gender, so it makes no difference to me how people perceive themselves. I use whatever name they use to introduce themselves, and only use pronouns when people aren't there, so am unlikely to 'misgender' anyone (with their knowledge, anyway). That doesn't mean that I am in favour of pressure groups with a vested interest in encouraging vulnerable young people to make decisions that can be very damaging. It is perfectly possible to be aware of the issues, supportive of people going through difficult experiences and also critical of groups such as Mermaids.