Very interesting sub-topic, which leads to us the counter-position of civil (non-marriage) unions:
Consider this: You meet a girl, convince her not to get married, but to live together and have children. One day thereafter, you buy a house, and place your name solely on the deed and mortgage of the house. The mortgage payment is $3,000 monthly. The other joint household expenses are $2,000 monthly. You and she agree because she lives there and gains the fruit of this lifestyle that she contribute $2,000 monthly toward the monthly expenditures. She gives you a check for $2,000 monthly and you deposit this check into your bank account from which you pay your mortgage.
10 years later, you and she break up. She brings a legal action, against you stating that you had both agreed that the house was jointly owned, but opted to keep her name off the deed and mortgage so you can both buy other properties' for investment reasons under her name, or she had potential litigious liability with XYZ so you both opted to keep her name off the deed for asset protection, or you and she had agreed to own the house together but since her credit score was lower, you both had agreed to keep her name off the mortgage (and deed as the originating lender required whoever is not on the mortgage cannot be on the deed) for a better interest rate. She requests the court to set aside the deed and create a new deed giving her 50% ownership--or that the court force the sale of the home and give her 50% of the net proceeds.
Now you have a legal action against you, evidence of monthly payments made to you, a 10-year relation with children, an alleged "marriage disguised as a civil union." Now the judge must render a decision based on case law, his personal bias, and the evidence.
Now let's say the judge rules for you. She now brings an appellate motion to stay the sale of the house, pending resolution of the "ownership" and the decision of the appellate court.
Now, after some time, the appellate court rules for you as well. She then commences a separate legal action alleging the the subject home was conditionally and jointly-owned by her due to your evasion of the "state marital law," and that you and she were in fact a "married" couple, and in the event the court finds you two were not legally married that the home ownership and sale is still subject to contention based on what serves the children's needs best--and that the court stay the sale of the home until this matter is properly adjudicated.
This matter can go and on ... for decades.
Point: Do not think because you did not sign a marriage license that you are protected from frivolous (and in some cases not so frivolous) litigation.
Choose your partner carefully.