Pica, really good to see you back. I've missed you.
My answer to your question, is that it matters to me. Men and women are treated differently. When I am treated as a man, and when I see myself as male, I suffer dysphoria. When I see myself as female, I experience euphoria. So from the point of view of my emotions, the male/female difference matters.
As for being happy, well I like being happy. Happiness buildings on itself, generating energy and good moods which make it easier for people around me to be happy. So in that respect it matters.
In the grand long run, nothing much matters. 100 years from now everyone who knew us will be dead and our record will slowly be erased from the earth. But the long run is not the only time frame that gets a vote. The present gets a voice as well.