"You're talking as though his time away from home is null time which is only of benefit to him. It's not."
billsykes - I say quite clearly in my post "Now presumably they've agreed between them that this makes sense because he is able to earn more money for them as a household unit." 
"What you seem to be saying is that if one member of the family is a high earner then the whole family should benefit from the money but the partner should expect to carry on as though they have a partner who works 9-5."
No, my point is that the high-earner as a parent is RELIANT on an amount of invisible, unpaid labour that is being done by the OP around the house, and that this can only really be fairly resolved if we stop thinking of wages as something individual, and start seeing them as something that is decided at the level of the household unit. As soon as someone is out of the 9-5 and not doing the 50/50 around the house, someone else is stepping in to pick up that slack. And that 'stepping in' needs to be seen as part of the higher wage - it enables the higher wage earner to do their thing.
This has nothing to do with free time. I am not saying the husband should be spending every spare second that he is home doing housework - the bargain that has been struck is not of that nature. It has to do with the monetary compensation that is provided to the person doing the enabling work of social reproduction. Further, the OP has also stated that her partner does get quite a lot of evenings of leisure when he's working away - I presume she's doing dinner, bathtime and bed at home, which isn't exactly the same kind of fun! There's something important about the nature of work here too: doing the same drudging routine every day is a different kind of tiring and disspiriting.
I think we agree that a decision has been taken by the couple with thought to the whole household unit. Parties are contributing different kinds of labour for maximum collective benefit. If that's the case, it means that thinking of salaries as "individual" no longer makes any kind of sense. (I also imagine that the OP's decision to change her job had a lot to do with a pragmatic need to maintain a household and childcare in the way that both parents wanted). In most families I know, a pragmatic balance is struck on the basis of something wider than individuals.