An estate plan can protect a long-term partner.
There are rarely provisions to protect people who are not married but are in long-term relationships. Many couples prefer to not get married. However, there still is a way to protect partners through an estate plan, according to the San Francisco Bay Times 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.
An estate planning attorney can advise you in creating an estate plan that fits your unique circumstances and may, or may not, include a marriage.
Reference: San Francisco Bay Times (June 21, 2018) "Want to Protect Your Partner? Marriage Isn't the Only Option."