"Oh and not living with both biological parents makes children miserable."
The study doesn't actually say that...
What it claims to have discovered is that children living with both parents are, on average, going to be happier than those who aren't.
"More likely" doesn't mean "will definitely", there will be plenty of happy single parent families and miserable marrieds out there.
However when looking at these figures it's worth considering something...
Single families will consists of a vast spectrum of "happiness" from the very happy to the very depressed.
Married families on the other hand will generally consist of just families that are the happy ones, as those that are miserable will often split up either because the marriage was making them unhappy or the other pressures that were will tear them apart.
So a lot of the "unhappy" married families that you might have otherwise counted have already split up and removed themselves from that category of the survey. And those formerly "unhappy" married families are now counted with the other "unhappy" single families.
So you've got a built in skewing of the figures that will move a large chunk of the "unhappy" results from one category to another.
And lo and behold that's what the survey shows.
Not cause and effect, but an obvious side effect of what they're counting.