This is so sad when it happens. It is a question that comes up a lot in online ask-a-lawyer forums. The question goes something like this: "My boyfriend owns the condo in which we have been living together for the past ten years. What happens if he dies? Will I have to move?"
The answer is an unequivocal, yes, you will be kicked out by your boyfriend's children or parents or siblings (whoever his intestate heirs are, which for certain does not include you), UNLESS your boyfriend has a will or trust leaving the condo to you.
A lot of people cohabit rather than marry for a variety of reasons, sometimes due to tax considerations (e.g., the so-called marriage tax), or to preserve social security benefits (maybe one of the couple is divorced) or because one of them is still married to another or some other reason. Whatever the reason, if you care at all about your partner, do him or her the loving thing and see an estate planning attorney to ensure that he or she is not left out in the cold if/when you pass away.
Remember, if you break up, you can always update your estate plan. Estate plans (usually) are not cast in stone.