Opportunity Cost and Parenting

We all say that we don't have time to do everything once we have kids, but what I think I find hardest is the opportunity cost when doing things.
Definition:

Opportunity cost is the cost of any activity measured in terms of the value of the next best alternative forgone (that is not chosen). It is the sacrifice related to the second best choice available to someone, or group, who has picked among several mutually exclusive choices.
Opportunity cost is a concept I came across when studying Economics (can't remember if it was during my GCSE or my A-level or my degree).

If my husband takes Aaron off to his Mums for the day, I never consider it "free time" as for each thing I decide to do, there is the opportunity cost of the things I haven't done.

So I have been known to spend the whole time they've been gone defrosting the fridge or freezer, etc... The last few weeks Aaron's still been in nursery 3 days a week despite my redundancy, and I always have to think, if I spend my time doing THIS, then THIS does not get done.

That's the thing, time is no longer an endless entity.  I have to weigh up what I can do when he is around, versus what I can only get done when he is not around.  It's constantly a game of weighing up and deciding.

I'm now in Ireland for 11 days, with Aaron for a big family wedding and yesterday my Mum took Aaron out for a free hours.  In theory I could have spent that time catching up on a book on my Kindle, but no, it was spent having a shower, getting dressed, unpacking his case, unpacking mine, and washing all the dishes.  Then it was time to go out in the sun (it was HOT yesterday) and catch up with them.

I have observed my husband and he doesn't seem to have this issue of "opportunity cost" and weighing and measuring out time.  If I put Aaron to bed at 21:30 and hop in myself at 22:30, the hubby will sometimes be up till 2, 3 or 4 a.m. watching movies, on the computer and generally doing his own thing.

Then Aaron and I will get up anything between 08:30 and 09:30 and if hubby is on a day off or late shift, he'll get up anything between 10 and 11 a.m.  So I watch him, and I think really, his time is still his own.  He doesn't have to fit around anyone, other than when we make explicit demands on his time, or requests for him to take us somewhere.

A long long time ago, I highlighted the differences between our parenting experiences and he simply said "well you're the primary carer!"

As I did not at all like that answer, I have never raised the subject again, for fear of hearing that response again.

I watch other couples and get glimpses of people's lives through their blogs, and I see a different style altogether.

What's your experience?

Like I say, it is not not having time to do things that bothers me the most, it is the constant balancing of "opportunity cost" that I find the hardest.