SEN. CHARLES MATHIAS calls it the "sin subsidy." Rep. Millicent Fenwick says it's a "tax on marriage." Whatever it is, the provision in the Internal Revenue code that lumps together the incomes of husbands and wives for tax purposes is taking a beating on Capitol Hill this summer. About a fourth of the House and a fifth of the Senate have signed on as co-sponsors of the Mathias-Fenwick bill that would permit married couples to file separate tax returns.

The tax policy this bill attacks treats the incomes of a married couple as if one person had earned all of it. This means a family with a $20,000 income pays about $2,745 in taxes regardless of whether one or both spouses work. But it also means that if each spouse earns $10,000, they pay $400 more than they would if they were not married. That difference, which gets larger as incomes go up, is the "marriage tax." It occurs because combining imcomes throws the second wage-earner into a higher tax bracket than he or she would otherwise be in.

This anomaly has existed for a decade, but the intensity of interest in it is new. Rep. Fenwick has a host of stories -- of the young woman who said she and her boyfriend couldn't afford the $668 in taxes a marriage would cost, of the minister who advised a young couple to forgo marriage because of the additional taxes, and of the female judge in New York who weighed the prices, in taxes, of marriage against the disadvantages of simply living with him. "There are 1.1 million non-sanctified relationship in this country," Rep. Fenwick says. "Government ought not to be pressing people away from marriage."

This issue would seem a natural for the Carter administration. It was President Carter, after all, who warned his staff against the evils of living in sin and urged marriage upon some reluctant associates. But the administration, while sympathetic, has yet to embrace the Mathias-Fenwick bill.

There are problems, you see. The first is money. Eliminating the "marriage tax" would eliminate $4 billion or $5 billion in federal income taxes. The other problem is more intriguing. As long as tax rates are progressive, some group of taxpayers will be shortchanged. It's either married couples with two incomes (as the law would be if Mathias-Fenwick prevails) or single people (if all married couples were allowed to spilt the family income).

The choice is not easy. But Sen. Mathias probably has the best of it with his argument that tax policy should "shore up the American family." That muchbattered institution is having a tough enough time as it is and needs all the help it can get.