What is the alternative for not sharing?

OP at one room, the couple at another. If the hotel charges by room, not by occupancy, each one is paying the same, therefore the couple is right to ask to share 50/50.

If the hotel charges by occupancy too, then the couple should pay a pro-rata according to what would be the rates for the single and double room. It won't be 50/50, but neither 67/33. Probably it will be something between 60/40 and 55/45. (I believe this is the OP's scenario.)

Finally, if they share the room, and the couple get a bed, while OP gets a sofa, then is expected the couple offers to pay a little more, because they have a better sleeping device. In this case, I think it will be fair and nice to them offer to pay 67/33 or even a little more.