For me BDSM IS sex. Even if no genitals ever received any contact, what I do with my master is sexual. From my perspective, your submissive friend's relationship with his Domme is sexual, it's just mental and sexual. Does he really separate it? Is he not getting off or getting turned on from those experiences?

I got spanked as a kid and I hated it. That punishment from my parents didn't scratch that masochistic itch at all. At the same time, I fantasized about being caged and locked up by ominous strangers, which turned me on, although at the time I didn't even understand I was getting turned on. So for me, masochism is sexual.

Of course it's not just about getting off, it satisfies a deep personal need too. But this is also true of vanilla sex for vanilla people. It gets them off, but also satisfies a deep need to feel connected to and loved by their significant other.