I'm not really sure, because I don't understand the concept of having a "secret girlfriend" or something like that. It feels dirty. Or maybe it's because I haven't separated "relationship" with its mono implications.
I think, it's because without noticing, I actually separated "boyfriend/girlfriend" and "relationship"... You tell OTHER people that you have a boyfriend/girlfriend, but not the person you're with because they are that person. But if you don't want people to know you're in a relationship with someone, and you talk to other people about them, they are your "friend". But that's lying and I refuse to do that. Therefore I don't have a relationship, but retain everything one entails.
I've also always despised serial monogamy. I've only ever been in a relationship with someone with the intention of being with them the rest of my life. It didn't exactly work out that way, which sucks, but I would never knowingly enter a relationship if I knew that it was never going to work out. So for me, being someone's "boyfriend/girlfriend" carries a lot of weight.
I suppose that's some insight into why I view it the way I do...but it's my own personal opinion shaped by my experiences, and it might not be the same for you.