If some women have penises and some men have vaginas, then some men will have babies, therefore it is a male thing to do.
But it is also a female thing to do. If everything can be done by both men and women, then living as a woman is indistinguishable from living as a man. Therefore no transgender person can be 'living as a woman' or 'living as a man', because there is no way of distinguishing between these two states.
In order for transgenderism to make sense, there must be things which only females can do or only males can do, so that a man can 'live as a woman' or a woman can 'live as a man'. What are those things?