I think the problem with your opinion is that it conveys that you believe children are being throw into surgery rooms and given sex transitions loosely and without thought to the consequences.
I think your real opinion is that you believe children shouldn't be given unnecessary surgeries. If that is true, the large majority of the population would agree with you.
In addition, have you explored what Doctors believe is a necessary sex transition for a child? What are those parameters? If you don't know, then I would consider your original stance to be of ignorance. Since you really never dive into the subject, most people will assume that you haven't explored those parameters and don't understand the reasoning behind them. That is why you're facing aggression with your opinion.