I'll answer, with the caveat that you should seek specific advice from a family law attorney. You should be able to find one with expertise in domestic partnership issues at QLaw, the GLBT bar association in Washington. They have a law clinic too - check them out.

http://q-law.org/legalclinic

Generally, domestic partnership works the same as marriage for all state law issues, so when you enter into one, you become a marital community (think of it like a business partnership) and your property and income, from then on, become community property. The stuff you had before remains your separate property. But, that paycheck you bring home every other week belongs to the "community" as does the things you buy with it.

So, say you had $100K equity in a $$200K condo, with a $100K mortgage, when you get not-quite-the-same-as-married. Maybe mostly your check actually goes to the mortgage, and your husband takes care of all the car payments and dinners out. Maybe you split everything down the middle, even steven. Maybe you take a year sabbatical to figure out what you really want to do, and your husband supports you. Maybe you two adopt your cousin Tanya's twins when she's serving time in prison, and your husband stays home to raise them while you take a most stressful job to pay the bills.

None of that matters. You each have an equal interest in the money coming in and debts flowing out of the community.

So, over the next ten years, you and your husband pay off the remaining $100K mortgage, and the condo is now worth $300K because the housing market bounces back. Say, all your other debts and assets even out, so the $300K house is what's left. You're splitting the sheets, so you're in court for the dissolution of your almost-marriage. You will have a claim for your original $100K, but you each have an equal claim on the remaining $200K - both the money that paid off the $100K mortgage, and the $100K increase in value during your marriage.

Remember too, that the dissolution of marriages and domestic partnerships are subject to the laws of equity - that essentially means that the court can and will use principals of fairness in splitting up your property. Did Husband give up his career to take care of the twins, and now needs additional education and training to return to work, and has lost ten years of "experience" so will be earning less? Did you put Husband through law school to the tune of $100K, and now he's making more than you? The judge can consider relative income, and divide the property in a way that gives Husband money to allow him to "catch up". Wait, did you two marry for two months, during which you got a huge bonus for work you did both before and after? Judge can consider that.

It's a lot to think about, especially for the couple where one has stuff, and one has debt, or one has education and one doesn't, or one is going to stay home with kids or work in nonprofit, and the other isn't. That's more of relationship advice. For legal advice - how you can plan, what you can plan, so that you both know what you're getting into - you should talk to a lawyer who can address your particular situation. Qlaw, FTW!