Families hit by the Government's "two-child limit" for welfare payments have started a High Court action against the policy.
The rule, introduced in April last year, restricts child tax credits and universal credit to the first two children in a family.
Lawyers acting on behalf of three UK families told London's High Court on Tuesday that the legislation was "detrimental" to children and urged a senior judge to declare it "irrational and unlawfully discriminatory".
Richard Drabble QC said the policy, created by the Welfare Reform and Work Act 2016, is "incompatible" with the European Convention on Human Rights and the UN Convention on the Rights of the Child.
He told the court: "The rule fails to have regard for the individual rights of children and is predicated on the proposition that those unable to support themselves solely through work should make particular choices about intimate behaviour."
"It marks a very significant reduction in the level of economic support provided by the state to lower-income children," he added.
"It affects children first not last, is discriminatory and is not temporary."
The barrister said the policy also affects more women than men and only tackles the "perceived problem" of lower-income families having more children because of benefits.
The court heard two of the anonymous claimants are single mothers, who each became pregnant "inadvertently" with their third child and are opposed to abortion, and their children.
One has physical and mental disabilities and Mr Drabble said that, in order to avoid any effects of the policy, she would have to earn at least £79,900-a-year - which he described as "fanciful".
He added: "Therefore the two-child rule does not provide any economic incentive to self-improvement, it just makes her family poorer."
A couple affected by the policy are also bringing the legal action, which is supported by the Child Poverty Action Group (CPAG), against the Department for Work and Pensions (DWP).
Mr Drabble said the policy "financially incentivises abortion and relationship breakdown" and that the choices facing poorer families were not the same as those for wealthier people.
"While a rich family chooses between surplus income and an extra child, a poor family chooses (to the extent they can actually choose) between meeting basic subsistence needs and an extra child," he added.
"It is irrational to describe that as 'the same choice'."
Lawyers for the DWP are contesting the case, arguing it was "not a policy aim" to influence parents' decisions about having children.
The hearing, before Mr Justice Ouseley, continues.