why did you decide to be friends with benefits? I don't understand your reasoning. If you're doing all the things you say you do, it's not like telling other people you're a couple (more or less serious), will change whether you'll get hurt or not.
I think the issue here is whether you or she are comfortable being seen as a couple. As usual in these situations, it has virtually NOTHING to do with the whole "things mustn't get serious, we mustn't get emotionally attached" rubbish because whether you tell people you're a couple who are just having fun or whether you don't, is not what changes how you feel.
If you choose not to be seen as a couple, it's because you might have an issue (i.e. don't want to be seen associated with that other person, might be somewhat ashamed).