IMHO we are free to think what we like. We should be free to say most things too, but some things we need to be careful not to say, or careful about how and where and to whom we say them to.
I would agree with that in certain more extreme scenarios. E.g. you probably shouldn't be yelling at someone in the street, "you aren't a real woman", because regardless of whether it's correct, it's just nasty behaviour.
However on social media say, it should be fine to express the opinion that trans-women aren't really women. You shouldn't be getting a police record for that.
Of course there are much worse things you could be saying. (Without implying that the above example is actually "bad" at all.)
Should you be able to share anti-semitic conspiracy theories at work? (During breaks say.)
Or should you be able to share completely accurate criticisms of the Islamic religion at work? (Which may be ultra offensive to some of the other employees who may hear the conversation.)