I think it's a case of they genuinely didn't go out back then as they had no babysitters to watch their massive tribe. A child free night out was the yearly dash to the hospital for Sue to drop the latest baby while her poor parents struggled to manage the rest of their rabble (this was even shown in an earlier show when she went in to have Casper, her mum and dad couldn't cope).
Now the eldest kids are old enough to be forced to give up their own weekends to watch the litter of younger siblings they never asked for, Sue and Noel seem to go out quite regularly just the two of them. Yet, according to them, they never expect the older ones to look after the younger ones...