But this calculation is ignoring the obvious – the couples are acting as a single buyer.
For example, let's say £1 buys 1 unit of space.
OP's £75 buys 75 units of space.
£50 buys 50 units of space. So, 2 people joining up their 50 units of space each are entitled to 100 units of space. Each couple has purchased more space than OP, in cold monetary terms.
It is unfair and it is the "single tax"... That's why I liked to share a room with friends even when single. Conversely though, sometimes when I go away with my boyfriend, we do fork out more so we both get our own rooms (we're both introverts so while we love hanging out together, we also like our personal space while unwinding on holiday)!
Anyway, back to the couples, they're benefitting from something called economies of scale (buying in bulk discount). Slightly controversial, but think of it like Brexit. When the UK was buying and making deals alongside 20+ member states, it got a bulk discount on many things so to speak. Now it's a single buyer, it probably won't get the same discount.
But from a non-economic viewpoint, I think an argument can be made for compassion. Not just for singles, but for example for less well-off friends, taking into account that the same holiday cost might be a greater % of someone's monthly salary than for others, etc.