Well! They have fairly the same concept!
Both involving having a relationship with a person(s), both involving having feelings for the person, both usually are pretty monogamous to the relationship (Less of course it's open but still then!), both have forms of counseling available. Normally you can have sex with either/or and are more than likely living with that person (At least my generation seems to). And both can end horribly with a Breakup. Of course with a Marriage when the person goes to *get their shit back* It's all itemized and filed away tidy-like in a Divorce.
Granted you get Tax cuts in a Marriage. xD