In a situation like that, if someone is after half your property and you are claiming that you were not in a relationship, they would need to prove the relationship existed. Which is pretty hard to do when it never did.
I haven't heard of any time when housemates gone sour has ended up like that, and something like that would be all over our stupid current affairs shows.
You don't apply, you don't opt out, simply when there are things where a married couple would get some type of benefits, for an example, Dita and Violet share car insurance, it works out cheaper for a couple than for two individuals, they just put Dita onto Violet's plan as her partner when Dita bought her car. You don't apply or not apply. It' just accepted as something that is. Only thing you need to prove is that you share a residence.