SAN ANTONIO - Registering for the draft has long been a minor rite of passage for young men. And only for men - but even those opposed to changing that rule say time might be running out on the last vestige of the barriers that once kept women out of uniform.

Defense Secretary Ashton Carter's decision last month to open up combat specialties that had been closed to women could force a such a change. For his part, the head of the Selective Service System, Lawrence Romo, said Thursday that his agency would be ready to sign up the roughly 6,500 American women who turn 18 every day, if Congress authorizes it.

"Some people come up to me and say, 'Hey, I think we should do it, it's the right thing to do,' " he told the San Antonio Express-News. "Other people say, 'Oh, not my daughter, not my granddaughter.' "

Romo, in a speech he will give Friday at the National Press Club, notes the Pentagon is working with Congress to "address the legal implications" on "the male-only registration requirement" as well as on how to implement the decision to open all combat roles to women.

Despite stresses on the all-volunteer force, few lawmakers have raised the prospect of reinstating the draft. But at least two legal cases, one in New Jersey and another in California, have challenged the Selective Service law's exclusion of women. A pair of lawyers familiar with the issue say the courts - which have historically deferred to the military - could base their ruling on Carter's decision.

St. Mary's University School of Law professor Jeff Addicott believes it's going to happen.

"From a legal perspective, the court … will have to order women to register for a draft," he said. "It may go up to the U.S. Supreme Court, but to open up all slots to women means that the (Defense Department) cannot ask the court to defer to their judgment as to who to exclude or include in terms of military service.

"In the past, the courts have allowed the military wide discretion in the criteria to exclude based on a number of factors," Addicott added, referring to weight, height, age, gender and sexual orientation. "But now the military have abrogated that discretion in terms of eliminating any position based on sex."

Registering for the Selective Service has been a matter of paperwork since conscription ended in 1973. But if women were required to register, and if Congress were to resurrect a draft, women for the first time would face a mandatory eight-year military service obligation, just as men have for decades.

Romo, a retired Air Force Reserve lieutenant colonel, did not take a position on whether women should register for Selective Service but said "women are doing very well in the Department of Defense" and "contributing immensely." Such a change would require 35 to 40 more workers and "some modest amounts of money" added to his agency's $23 million annual budget, he estimated.

The issue could become part of a larger debate on Capitol Hill over Carter's recent order, Romo said - or it could be decided in one of the current court cases. He said the Department of Justice has asked a California judge to withhold a decision in one lawsuit so Congress can act first.

One critic expanding the role of women in the military pointed to a 1981 Supreme Court ruling that said the primary purpose of a draft in a wartime scenario would be to provide for combat replacements, and that "there is no compelling reason to include women" in that system since few would likely serve in such roles.

Carter's policy, however, "threatens to knock out from under" the rationale for women's Selective Service exemption, said Elaine Donnelly, president of the Center for Military Readiness. "If decisions are based on military necessity and not 'equity,' great numbers of women would not meet the applicable minimal qualifications for the infantry."