Problem is, without a wedding there isn't a bond or promise that holds the two together. There's nothing to work for. Sure, there will be problems and there won't ever be perfection (no matter how hard they try to achieve it), but that never happens, even outside of weddings. And without that promise and bond, it's sure to end eventually. If there's anything I've learned by watching people around me, it's that anything and everything good takes work.
Weddings break apart by divorce. It's a sacred bond that's broken and leaves both sides shattered. It's pain all the way through when two people get divorced. But without marriage, the ending is never satisfactory.
...And I really need to go. Bye!