Sauwelios wrote:"The hallmark of the slave in BDSM is that (s)he chooses 'once and for all' to submit unconditionally; whereas the hallmark of the sub is that (s)he chooses at every moment whether to submit or not."
If this is correct, then a married couple may be said to be each other's _slaves_, whereas an unmarried couple may be said to be each other's _subs_. Or, inasmuch as being one another's slaves or subs is absurd, a married couple may be said to be slaves to their relationship, whereas an unmarried couple may be said to be subs to their relationship.
One might say that the employer is slave to the worker, and vice versa. After all, the employer needs work done and the worker needs a paycheck... so they're tied inescapably to one another.
Or that the listener is slave to the speaker, and vice versa. It's rude to walk away when someone is speaking, or to ignore someone who asks you a question.
In these sorts or "non-literal" senses we are all slaves to one another. The unmarried couple as much as anyone else.
In terms or real slavery, however, I don't see the analogous parallels. In today's world, divorce is far too common; and even when marriage was forever, there was still plenty of room for compromise. As part of this, the woman was typically the 'master' inside the house. Within limits (such as what could be afforded) she chose what happened in the house... and even today we still see the remnant of this (although it's not nearly as obvious as it once was) for when a man and a woman get married (or even move in together) it is more typical (I suspect) for the woman to chose which things stay and which things go. Hollywood doesn't always reflect reality very well, be we see it in movies all the time where the man wants to keep his lamp or his chair, and the woman says NO! It's ugly. All the man wants to keep is one lousy remnant from his former life and she feels it's her right to trump him. Conversely, the man was the master outside of the actual house. He worked the fields and voted in politics and so forth.
So even then, while one might suggest there was a slave-master relationship, it was typically reciprocal in nature, with each side having mastery in only a limited area of daily living. Today, those limited areas of mastery have been greatly blurred, and (in many cases) the marriage union is no more permanent than the unmarried union. The unmarried couple are more likely to live separately. But other than that, I don't see any particular increase in the [slavery] analogy for a married couple. Each is free, at any point, to make decisions--although there can be repercussions for those decisions. But that's equally true for any 'cooperative union". The worker or the employer can do lots of things, but some of those things will get you fired or sued. And others will just make the working environment less enjoyable or productive, etc. Which will encourage one party or the other to explore other union options. Marriage is essentially the same. It's no more slavery than any number of other human interactions.
