There are many that claim that women are held back by the expectation to take care of children, and that affects their career opportunities and choices.

This shows the demographics for all women above the age of 16, along with how many have children under the age of 18, and split in a number of age groups for children, and their workforce participaton. The rightmost column showing that 48.8 million women are not in the workforce, with 38.3 million of that having zero children under the age of 18. Now this includes retired women though, so it would unfair to just say "80% of women not in the workforce don't have children to take care of". Women over 65 make up ~7% of the population, which is ~22million. Now, it's a pretty safe assumption few if any woman over 65 has children under the age of 18. So subtracting 22million each you get 26.8 million women aged 16-64, of which 16.3 million have zero children under the age of 18.

That makes it 60.8% of women 16-64 that are not in the workforce and have zero children 18 or under

Now, let's look at just married women. 23 million married women 16 and up are not in the workforce, of which 15.2 million have zero children 18 and under.

Here shows the breakdown of what percentage of women 65-74, 75-84, and 85+ are married. Here shows the population breakdowns of men and women by age. From these we get:

65-74: 11.2million, 57% married=6.4million married

75-84: 7.5 million, 37% married=2.8million married

85+: 3million, 15% married=450,000 married.

These added together yields the number of women over 65 that are married=9.65 million.

Subtracting that from the married columns for those women not in workforce and without children we get

13.35million married women not in workforce aged 16-64(obviously there are few if any married women younger than 18, but "noninstitutional" means 16 and over), of which 5.5 million have zero children under the age of 18.

This yields 41% of women aged 16-64 not in the workforce are married with zero children under 18

There could be more accurate percentages with more data regarding distribution among age groups, but there appears to be a strong indication children are not the reason women are not in the work force as much.

There is something to be said about 40% being married with zero children. It wouldn't be unreasonable to conclude that women are opting to be married to not have to work, but I think that might be premature to conclude with just this data. Investigation including the breakdown among part time workers would clarify this further.