Let’s consider for a moment a separate group of custodial parents: those who weren’t awarded child support in the first place. There are lots of them to look at — nearly half of all custodial mothers and three-quarters of custodial fathers don’t have a legal agreement for child support payments in place. When asked why, custodial fathers are twice as likely to say “child stays with other parent part of the time.” They’re also slightly more likely to cite financial reasons like “child’s other parent provides what she can” or “child’s other parent could not afford to pay.”

http://fivethirtyeight.com/features/are-moms-less-likely-than-dads-to-pay-child-support/

That’s not all. The average household income of a dad who doesn’t get the child support money he’s due is $51,791. For moms, that figure is $26,231.

Easier to say when living a bit above the poverty line.

If the mothers had abortions, the father wouldn't have to pay, sure. But apparently it's not common that men who can't afford to pay child support actually do. If you don't have the money, you can't simply can't pay! But if you don't have an abortion, the child is still there, needing to eat, wehether or not mom or dad can afford to feed him/her