I've only ever had one SO, but there are plenty of things I don't tell him about previous people I had feelings for. If he asked outright, I'd give him the bare minimum, but I probably wouldn't give the full extent. There's no need for him to know. If he and I ever break up, there will definitely be private things between us that I'll never tell anyone else. I don't see what's wrong with that. Don't want to hurt anyone's feelings, and some things should remain completely private.