The issue of trying to be full time is admitting you want to have a sexually charged lifestyle. D/s allows you to be reminded of things sexual at all times. All too many vanilla relationships die for lack of excitement.

Mere control/ownership smacks of being property... which creates no sexual tension and leads to a similar boredom.

So if you agree with those statements, then a full time relationship indeed becomes "BDSM activities [in] the bedroom (or wherever they have sex)" because you are in fact "having sex," at least mentally, in all the non-bedroom venues in which you are practicing D/s.