I must disagree. It is only slavery for those who chose for it to be. Marriage can be an equal partnership. What it is just depends on what kind of relationship the couple decides to have, both as individuals and as a pair.
In a way I do and in a way I don't. Back in the day it sure as hell was, but today the context has changed. You're not a literal slave anymore, but you still are institutionalized. Why marriage can be a psychological form of slavery (and not everyone agrees with this), is because by nature humans were never designed to handle marriage. Marriage, no matter if it's polygamous or monogamous, typically has its participants confined to one person or more for long periods of time. Humans are not born being able to stand this. They're polygamous and don't like staying in one spot with the same person(s) for all their lives. They get bored of it. So having to do that for twenty plus years can cause strain between a person and their mate, or mates, because they don't have the mental capabilities to handle the situation. In that way, marriage can feel like a prison or a form of slavery. Because 'you' hypothetically have lost your sense of freedom. Take marriage out of the equation, and you're no longer 'bonded' anymore to anyone. You can do what you want (or who you want, if I may be so blunt) when you want.
The exceptions the feeling of bondage in marriage are instances where marriage is either open or doesn't exist at all. There's a culture in China for example that expresses (very loosely in my opinion) 'free love' and a lack of institutionalized marriage. They have informal ceremonies that might be considered as weddings to outsiders, but really they're only little get-togethers for the sole purpose of allowing the matriarch of the household to decide whether or not a certain man is worthy of dating her daughter or not.