I've been in a relationship for 15+ years and never married. I made the decision not to do a domestic partnership or get married. My partner and I keep everything separate. However, we both have successful careers, good insurance options, and are financially well of. A domestic partnership adds some benefits, but I would think about marriage if you have medical issues and have concerns about your partner if you are gone.
Big thoughts.
Marriage or DP:
In your case, I would think about marriage, unless you have some moral objection. You live together, have commingled funds, and mainly married without the legal standing. A domestic partnership may help in most of the smaller life things (insurance coverages primarily). However, if you have a medical issue, marriage will aid you in death more so than a domestic partnership. Your partner will have more rights and be able to better care for you.
Will & Trusts
Why don't you have a will? This is much more important than a domestic partnership or marriage. I would set up a will & living revocable trust since you aren't married. If you intend to live similar to a married couple, it makes sense to have wills/trusts for one another, children, or other loved ones.
Life Insurance
Insurance: Life insurance is a big issue. I don't know your condition or insurability. However, you guys should consider term if you are financially dependent upon one another. It makes sense to have 10x your income in term for the surviving party, so they can make the transition easier.
Taxes:
The marriage penalty is real. If your combined incomes reach over $150K and you don't have kids, you will most likely pay more taxes combined than separate.
Divorce
I've dealt with domestic partnership separations and they are a nightmare. I would consider marriage for the simple fact that it clearly defines your relationship, assets, and rights in the eyes of the law. 17 years of a relationship would be enough ammunition for each of you to make claims on the other's assets in a separation. May not happen if you break up, but I would get married at this point for no other reason than making it legal.
Children
If you have children or plan on it, marriage makes parental rights much easier. Same with adoption. I wouldn't do a DP if I was considering children as it adds an unnecessary layer of complexity.
Lastly, if you guys plan on a DP, I would get the will & trust documents set beforehand. They are much more of a priority than defining your relationship.