You don't want to get married simply because you have never liked the idea of marriage as an institution. That does not mean you're not committed to your relationship. You are. You and your partner stay together for 10 years.
Then it finally ends. You've been living together and sharing your life just like a married couple. If you break up, do you have property rights just like you would in a divorce?
Unfortunately, the reality is that you often do not. The court doesn't think of you as a couple because, legally speaking, you're not. The court isn't making a ruling based on how you felt about each other or what labels you used. They just look at your legal standing, see that you are two individuals, and rule accordingly.
This can make for a very complicated situation. For instance, there are cases where you can prove that you both had a "common intent" to jointly own the property. Maybe your significant other bought a home and you helped pay the mortgage. Half of the down payment came out of your bank account. You think of it as your house, too.
In some cases, you can prove this, but experts warn that it is very difficult. If you and your significant other did not have anything in writing, it could just be your word against theirs. If their name is on the deed and the mortgage papers, you may not get anything.
The best thing you can do is to plan, in advance, and get your paperwork in place. Either way, though, you always need to know what legal rights you have.