In the same-sex marriage debate, one major issue that gets short shrift is the impact of U.S. tax law on same-sex couples – specifically, how it affects their children.

U.S. tax law allows heterosexual married couples to file joint tax returns, enabling them to pool their income so they pay less in federal income taxes and have more money to support their children.

But same-sex couples with children cannot maximize tax benefits in this way. They can’t aggregate their incomes and divide the amount, which results in less money for their children.

This is why I wrote a few weeks ago that it was critical the Senate approve President Obama’s nominee for the Justice Department’s Office of Legal Counsel, Professor Dawn Johnsen. Johnsen is the kind of person who would opine that such a policy violates the Fourteenth Amendment and who would advocate that the Treasury Department must allow individuals in same-sex unions to file jointly if doing so would benefit the children in the household.

After all, raising a child is a huge expense. It benefits the country to provide some support to those who raise the next generation of taxpayers and contributors to Social Security and Medicare. Yet about the only relief same-sex couples have received from tax officials came in 2008, when the U.S. Tax Court issued a non-precedent-setting decision allowing a woman to claim her same-sex “friend,” who lived with her, as well as the “friend’s” grandchildren, who also lived with them, as dependents on her federal income-tax return.

Lesbian/Gay Law Notes reported that the ruling was not that unusual because the taxpayer provided the majority of the financial support for her friend and the friend’s grandchildren, since she’d earned almost $30,000 that year compared to her unemployed friend’s $8,000 in disability benefits. (The children’s parents had disclaimed responsibility for supporting them and were not claiming them as dependents.) In addition, due to her low income, the friend did not file her own tax return.

The Tax Court found it was appropriate to allow the taxpayer to claim dependency exemption deductions, but if it had been required to consider issues of same-sex marriage, it would have found otherwise, because same-sex couples cannot file jointly.

Is this fair? Obviously not.

Nor is it fair that gay singles, who can take a dependency deduction for their minor children, may be at a loss when trying to deduct expenses for assisted reproduction.

For instance, in another case, also reported by Lesbian/ Gay Law Notes, a gay man appealed the Internal Revenue Service’s refusal to allow his medical deduction for expenses incurred in the conception and birth of his two children by gestational surrogacy. However, the Tax Court ruled, in Magdalin v. Commissioner of Internal Revenue, that the expenses were not deductible because they did not involve treatment of any medical condition suffered by the taxpayer or intended to affect a structure or function of the taxpayer’s body.

Magdalin had fathered two children without the use of any reproductive technology during marriage to a woman, thus showing he had no fertility problems that would require medical treatment. After the marriage ended, Magdalin contracted with other women to be gestational surrogates, resulting in two more children. He then documented his expenses as medical deductions on his federal tax returns to the extent they exceeded the statutory floor of 7.5 percent of his gross income for each year.

But the IRS disallowed the deductions, and Magdalin, representing himself, appealed to the Tax Court. Referring to an IRS letter allowing such deductions for a married couple, he argued that it was unfair that a married couple using gestational surrogacy because of a wife’s inability to conceive or bear children could deduct the expenses, but that he, an unmarried gay man, could not do the same.

Nevertheless, the Tax Court found that the tax code allows medical deductions only for “medical care of the taxpayer, his spouse, or a dependent.” This policy discriminates against individuals who cannot reproduce on their own – even heterosexuals. Assisted-reproduction expenses will be necessary regardless of whether the individual is single, gay, heterosexual, married or in a same-sex union and they should be allowed as medical deductions.