Totally agree OP.
The issue with a partner is lines get very blurred. If you do move in and subsidise rent you need terms written down somewhere
I am friends with a couple who are in the process of splitting up. Going to explain and try not to apply gender, as my DH and I are both friends with one of the couple and we cannot work out what is 'right' and we suspect there is no clear answer in the absence of marriage (hope this isn't hijacking the thread but I think it's relevant - just report comment if it bugs you).
Partner A owned a house for 3 years before the start of the relationship and put down a 30% deposit. When they got together (they had been friends for a long time) B was trying to buy a flat but in the end they moved in together to A's house because they couldn't rent anywhere of a similar size for anything close to the mortgage (London).
They have done 2 rounds of work on the house, funded 75% by A and 25% by B which has significantly increased value. Of course, when they were doing this work they were talking about it being their 'forever home' and where they wanted to watch their children grow up.
They lived together in the property for 4 years before having DC (1 DS).
They are not married.
B used to pay half the mortgage and bills (which in total was a lot less than they paid in rent alone), but for past 2 years has paid nursery fees for DC, which is actually higher than mortgage. They both pay 50% of bills.
B believes the house should be sold, deposit and first 3 years of equity returned to A and they should then get half the equity each from the date B moved in. A believes the house in in their name only, B had no deposit to buy anywhere before moving in and therefore B was renting and is not entitled to anything. We are talking about very large sums of money.
This is the problem with paying a partner's mortgage - one person feels they are investing in their future as a couple, when in reality if they split, they (may) become 'just a tenant'.
(We've told them both to talk to solicitors but whatever the conclusion they will probably both be unhappy and forced to compromise.)