3. If you break up, there's an easy way out.
Let's face it: the majority of relationships end. People change, and sometimes they don't change in the same directions. If you're married, exiting your relationship requires, at the very least, legal processes and filing fees and mandatory waiting periods, sometimes of up to one year.
Think about your last breakup. How'd you like to drag that out for a year? However, if you never combine your finances or your households in the eyes of the law, exiting your relationship requires nothing but some introspection, conversations, tears, and a few cases of wine.