If you have a long-term partner who you want to make sure is taken carry of after you pass away, getting married is not your only option. You can get an estate plan.
However, there are no provisions for people who are not married but are in a long-term relationship. That is one reason why the LGBTQ community and its allies fought so hard for gay marriage. Today, many heterosexual couples do not want to get married and many LGBTQ partners would rather not as well. Regardless, there still is a way to protect partners outside of marriage, as the San Francisco Bay Times discusses in "Want to Protect Your Partner? Marriage Isn't the Only Option."
The default rule of intestate succession in estate law is not the only option. Most of the benefits of marriage can be had by other couples, as long as they plan ahead. With an estate plan, you can direct that your property go to your partner after you pass away. There are even ways to make sure your partner is taken care of for life, but then afterwards someone else ultimately inherits any remaining property.
Marriage is not for everyone. If it is not for you, it may be wise to speak with an estate planning attorney about how to make sure your partner is protected.
Reference: San Francisco Bay Times (June 21, 2018) "Want to Protect Your Partner? Marriage Isn't the Only Option."