For many people sex is the focus for their BDSM activities. However, other motivations are frequently involved and these may or may not be obvious to the person involved. This is because sometimes people do BDSM or are attracted to BDSM for reasons of which they are not aware. This can simply be because they've never realised it, or can be because they are uncomfortable with or don't want to accept that they have these particular drives and so they hide the fact in a corner of their mind and pretend it's not there.
Getting back to sex for a moment, consider the following extract from a personal advertisement:
I love male/male bondage and discipline, specifically: gags, hoods, blindfolds, leather, ropes, collars, leashes, chains, flogging, spanking, nipple play, etc.
It's clear that sex wasn't the focus of this gentleman's interest. True needs and desires often surface most clearly when someone is advertising for a partner.
This is not to say that sex isn't important, useful, and significant in a BDSM context. Sex is an ideal activity to satisfy the penetration aspect of BDSM, and can increase feelings of intimacy and bonding. Indeed, sex can be combined with other penetration activities to give a double or triple whammy-type boost to sex.
Overt sexual activity, such as intercourse and fondling, are limited to private contexts, such as personal dungeon or the bedroom, but less obvious sexual activities---such as not wearing any underwear, or trimming one's pubic hair, or wearing revealing or sexy clothes under orders from one's dominant---can have an effect both in private and in public.
The focus on sex by many BDSM players is what leads fetish and BDSM to be blurred together in many people's minds.