I actually feel a bit sorry for the girlfriend. She may have refused the marriage proposal for a myriad of reasons; they weren't ready, settled, financially able and wanted to wait a bit longer or maybe, just maybe, she didn't feel marriage was something she wanted to do - not because she devalues the relationship, but because marriage isn't her thing (although you'd assume they would have had that discussion). Maybe they were going through a rough patch, and felt now wasn't the time? Maybe the boyfriend was proposing as a "get out of the doghouse" card - there are just too many what if situations.
6 years together is a good while. I appreciate the aunt may fear that giving the girlfriend equality in the house may allow her at a further date the opportunity to benefit - would the aunt feel differently if they were married? If so, why? Marriages break down all the time, and people lay claim to all sorts; them being married doesn't prevent it from happening. I would feel snubbed, unless there is a back story of the girlfriend behaving in a certain way, if my boyfriend's mother offered to buy a house and prevent me from having any kind of ownership on that properly and simultaneously expect me to live there without batting an eyelid over my security, I'd assume she didn't rate me much as a life partner for her son. I appreciate it is a gift to her son, but surely any kind of bill paying or alterations would be from a joint venture/both parties, rather than just one? I certainly would not be able to see a place as "home" if I felt that at any one time, the relationship could end and I would be asked to leave - the house is his, so that would be the case; it's unnerving and would probably be detrimental to anyone in that situation, more so because the aunt has asked for it to be that way.
My Mum was in a long term, stable looking relationship with someone after her divorce from my Dad - he bought a house in the village she had lived in after selling his own and that was that. She had no claim on the property, but was grateful to have somewhere to live. My Mum, although not on the mortgage, paid for several alterations to the house, as well as bills and upkeep. Her (now ex) partner, upon a windfall from a relative dying, decided to ditch my Mum in the most inhumane way and left her both destitute, homeless and at the mercy of friends/family to ensure she got through. He did it to prevent her from getting his assets, and had she legally been on the property which has now sold for twice the value it was bought, she would have had at least some money to either put towards a retirement plan or purchase a house, rather than living at the age of 53 in a council rented, tiny one bed flat with no garden. My Mum was foolish and admits it, but just living in a property with someone who claims to love you does not always guarantee you security, sadly.
That said, there are some shitty people out there who will exploit whatever they can. So I do see the aunt's POV, completely.