I totally identify with this.
I've never felt shame that I was adopted, and it's always been clear in my mind that anyone who judged me negatively for it would be someone I didn't want in my life anyway - the kind of person who would assume that I had abandonment issues or that any time I did something wild or irrational that it was because of my childhood.
But I also know most people, even if they don't judge negatively, will turn the fact that I'm an adoptee into a Thing. Because it's unusual, they probably can't imagine that sort of upbringing and they will define me by it in some way. In reality I don't consider it an interesting part of my identity. The thing that sometimes disquiets me is the absence of any feelings I have about it, not the strength of those feelings. It just was what it was.
I don't especially want to discuss any of the above in depth, either. And I know talking to people about being adopted will open that door.
On top of that there's finding an appropriate conversation to drop it into! It's not something I feel I have to sit someone down and "confess" to, but it's also not really something you can casually mention as an aside.
That's why it's not information I tell all and sundry. However on the subject of the actual thread, I told DP fairly early on when we were dating, and I did in fact drop it into the conversation as an aside, because I was a bit drunk and we were having a bit of a heart-to-heart anyway. He's always respected the fact that I don't have much else to say about it and treats it as the non-issue I'd like him to.