I do think that both DC and DP have a legitimate reason for preferential treatment. Neither is currently paying a mortgage, so both need to set some money aside.
One's child, I can see. Parents often help our their children in getting on the property ladder in one way or another. But why on earth should one adult facilitate another adult saving for a mortgage by disadvantaging themselves financially?
Maybe I should tell my nephew that he needs to tell his roommates that he's going to only pay half the required share so he can save more for a mortgage.
As far as paying an amount that might be considered 'contributing to the mortgage', OP definitely needs to find out the legalities of that. Certainly there have been any number of posters who have been tossed on on their ears by ExDPs who are sole owners and have been told they are SoL by solicitors, as well as posters who were planning to claim 'home rights' due to being unmarried, having shared minor children and contributing towards the mortgage. But I honestly cannot say that I remember any posters saying "I contributed, we were unmarried, had no shared minor children, and I was given a share/joint ownership/whatever".
I'm married to a wonderful man and we're now retired. Let me tell you one thing...if I were to become single it would be a cold day in hell before I moved a man, any man, into my home. I wouldn't care if he was rich as Croesus or had a golden cock. There will be no boots under my bed, no siree! It's just not worth it.