It highly depends on the people involved, and the reasons they have for doing it.
To me, it doesn't feel like I'm doing something unnatural or being forced into anything, it doesn't even feel like roleplaying. If anything, playing a relationship out more normally feels like I'm roleplaying, as that's not how I'd be naturally. The "bdsm relationship" to me is more of a discussion of things we like and are comfortable doing, and from it our own body language seems to develop alongside deeper, more honest discussions. I like pain, I like being dragged around, I like being bossed around when it's done right, so why wouldn't I want that to be outside of the bedroom? Part of my brat-like behavior is to elicit those responses, wanting them to give into those violent impulses for my own benefit, take control of the situation for my own enjoyment. Before I even really knew much about bdsm I still had who I was with at the time punching me as a form of flirting, because that's what was natural between the two of us after getting to know one another.
I don't even read it like a bdsm relationship, I read it more like a relationship that reflects the things that we truly enjoy. Why limit myself when I can have what I crave instead? The only times that it feels a little more above and beyond normalcy is when I'm wearing a collar in public and being leashed around from place to place, and that's more of a thing that's (currently) reserved for conventions or events. It's a lack of pretense when compared to a life without these honest expressions of the self. It's liberating to be with someone who gets me that I also get back, expressed through compatible kinks and lifestyle preferences.