I'm a christian, so I'll answer this in a christian's perspective. if you really are sure about your partner, not getting married would be a selfish thing to make. Not getting married shows that a person cannot commit ever. Getting married will solidify the partnership, thus two flesh becomes one.
If both are just thinking of their individual selves, then they both will not possibly able to commit in a partnership. It's like an accident waiting to happen. Each individual just anticipates and waits for someone to commit a mistake and then that's it, it's over.
They will say, what's the use? the spark isn't there anymore. Love isn't something like food that when you're fed up eating it, you stop eating.
The spark is only necessary for an engine to start running. While running, don't expect the spark to be there, you just keep on driving.
When you're fed up eating a hotdog, you don't throw it away, you could put a ketchup on it, and a mayonnaise too.
The advantage of it is it strengthens your partnership, make a more unified family, you'll have commitment to each other and to your kids. Together, you can do things better.