Science. It's not about disagreeing with it on some moral level, but rather how it's potentially not rooted in fact, but rather in fantasy. @NotAzizele said that I can say my thoughts and no offense will be taken, so let's not get into a heated argument... just wanted to point out that it's not always about being intolerant or fearful. Sometimes it's just about the proof in the pudding.
Most scientists agree that there's not enough proof yet to support that transgender is an actual thing. For me, I'm just wary about supporting transitions until it can be better supported by science. Suicide rates are still very high among transgendered people, post-op, or post-transition, so you've got to figure that there must be more to the problem.
That being said, I'm not against anyone's rights, or anyone's happiness; I'm just concerned with the possibility of jumping to potentially incorrect conclusions.
Be safe, and be happy.