This is how I expressed it as part of a "M/s FAQ" on my first website:

How can you keep someone as a slave in this society?

Legally you can't. There is no way for someone to enter a legally binding obligation that surrenders their basic freedoms. Even the Army, which used to be the nearest thing to legalised slavery left in our society, has had to give people the freedom to leave if they find their treatment intolerable. Likewise there is no way for anyone to remove another person's basic freedoms without due process of law. On the other hand, anyone is free to choose to live as if someone else had absolute power over hir.

So it's only a game?

Football is "only a game", but millions of people shape their lives round it and a few people kill for it. Most passions seem absurd to those who don't feel them. For those of us who feel the need to serve or be served, the Owner's power and the slave's subjection are as real and as inescapable as if I really had the legal right to beat my slaves to death if they don't please me.