Life is all about choices. A submissive person always chooses whom to serve. She/he can choose to serve any master that comes along or choose just one to serve. If the master she/he chooses to serve has her/him serve another or others, she/he is still choosing whether to serve or not. This isn't actual slavery, which is illegal in most (all?) countries. BDSM slavery is consentual and a fantasy brought to life, so it's what you make of it. Choose wisely.