OK, I'm no longer specifically talking about the word "period", I'm talking about all the words pertaining to members of the female sex and their sex based needs and bodily functions which have been redefined to include members of the male sex.
Take the words "woman" and "female", as the most basic example.
Imagine you were raped by a man. You join a women only rape crisis group for female survivors of rape and sexual assault. One day a male person joins the group. This sets all your trauma responses off and you are no longer able to participate in the discussion. You ask the service provider if it is possible to move to a women's group which is exclusively for female survivors only. They say no, because trans women are women, and also female.
How do you stand any chance of getting what you need, or even saying what you need, if the people you are talking to are using the words "women" and "female" to include the literal opposite of what you understand these words to mean, and when even when you understand the other person's different perspective, there are no other words you can actually use to communicate your meaning because they have all been taken and redefined?
This isn't about period pants in isolation.
It is the appropriation of womanhood and women's issues by biological males who believe that female people's language, spaces and even healthcare communications should be dramatically altered to include and centre them.
So yeah, it is actually important for there to be clear words which society uses to label particular things and people, particularly things and people as important as, say, the entire childbearing sex, and that we do not tolerate the use of these words being changed to include their literal opposite.
And for what it's worth, fetishising periods is fucking grim and people who do this should not be pandered to.