I've been in relationships where I haven't told people, mostly my parents, that I'm in a relationship. It's not in an effort to keep it a secret from them, but just something I'd bring up when I know the relationship has been going on for a good length of time to finally tell them or when I see things are getting more serious. I don't want it to be like I end up in this new relationship, tell everyone about it, and then a week later it's over. That's happened before.
In 7th grade, however, I had a girlfriend who I had been with for a good while, but we couldn't tell her parents because they wouldn't allow her to have a boyfriend at the time. Unfortunately, living in a small town where everyone knows everyone, that didn't stay secret for long and we had to break up because of it. We stayed good friends though.
Ultimately, I'm indifferent about the topic. I feel like if you love someone, why hide that? If you have a good reason to keep it a secret though, then I respect that. However, there comes a time when a relationship just shouldn't be kept a secret anymore otherwise why are you in a relationship with someone you can't admit publicly?