It is required not to be unfair, while it preferable to be reasonable.
You don't need to demand people work at your level, but you do need to demand that they work at a level that they can do if you are working at a higher level.
This all goes back to all the recent posts about lazy people in offices should be allowed to be lazy and unproductive.
Sure - within reason.
on edit: the demand that they work at a level that they can do only pertains when it is a task you care about - like child rearing. If one member of a couple is not able to contribute as much but does not contribute what they can this would create a rightful and detrimental resentment.
on second edit: changed if one couple to if one member of a couple otherwise didn't make much sense.
It would be best to realize that not everyone is lazy just because they won't deliver the same ammount of work units as you. We are all different, with different needs and handicaps.
1. Be patient. No matter what.
14. Learn the needs of those around you and respect them.*
The problem is I can deliver a lot and others can't so I quite often expect more of them since I expect them to deliver as much as me.