Sexuality/romantic stuff. I'm bi, but I never talk about it, unless it's relevant or to encourage someone. I'm married anyway, so that time of my life is passed.
Also bi, also happily married, my partner is supportive, but it hasn't really changed anything in our lives. So while it's not a completely moot point, I don't really share this part of myself outside of similar circumstances.
I sorted things out later in life. So to be honest it's still a little strange for me to talk about it online, but it's definitely part of who I am, and I have to wonder how many other people out there are living like this.