It is about beliefs. If they want to pretend they are the opposite sex, to the point where they end up believing they are, they can do so. If they want to take medication in an attempt to imitate some opposite sex characteristics, because they believe it turns them into the sex they desire to be, they can do that too. Not a problem.
However, if they want to use services and spaces intended only for the opposite sex, that is a problem, and it is where the conflict lies. For women and girls in particular, this takes away their dignity and safety, if any man who says he's a woman gets to use women's spaces.
That's what the men who smashed up that feminist conference (as described in the news article I linked above) were protesting. Women saying no to them on this.
I'm not straight by the way but it wouldn't matter if I was. Not sure why you made that false assumption or thought it relevant.