And why is there such a strong MN attitude against trans?
There isn't, hazeimcgee. There is a strong attitude against women being told that they are no longer entitled to women's facilities, or women's services, and that people with penises have rights that always trump women's rights, even in women's space. You might like to know that some transwomen agree with us on this, and these transwomen suffer a lot of abuse from extreme trans activists because of it, just as some feminists do.
Surely we're all free to live our lives how we want if it doesn't cause harm?
Yes, I agree! But allowing people with penises into sex-segregated women's public and private place directly does put women in danger. The most benign outcome is that women will withdraw from those areas of public life (gyms, public building, etc) where they can't have privacy from people with male bodies. This reverses hundreds of years of women's rights. The worst outcome is that incredibly vulnerable women who have no choice, eg women in domestic violence shelters, prisons, etc, will be at serious and continued risk of extreme mental and physical damage. There already is harm being caused - some of us here think that there has to be another way.