WHY IT HAS taken nearly a year of bureaucratic backing and hauling we may never know -- but finally the Carter administration has stopped dredging up strained defenses of an outlandish immigration policy and has reached the obvious conclusion that a ban on the admission of homosexuals to this country should be repealed. The prohibition was a bad idea when Congress first wrote it onto the books 28 years ago, and in today's world it is all the more anachronistic and pointless. Yet even in joining those favoring repeal, the administration could take a far more forceful stand than it has.

The White House decision consists of support for the objective of legislation introduced by Sen. Alan Cranston (D-Calif.). This bill would delete a reference in the Immigration and Nationality Act to "sexual deviation" as a condition under which immigration officers are required to bar admission of anyone suspected of being homosexual. o

It was a series of absurd "suspicions" and enforcment decisions that led to an announcement last summer by the surgeon general that homosexuality would no longer be considered by government physicians to be a "mental disease or defect." The Public Health Service said it would no longer accept referrals from the Immigration and Naturalization Service to determine if aliens were homosexual. That prompted INS to seek an opinion from the Justice Department, which concluded that the old statutory ban had to be enforced.

Even with White House support, the Cranston legislation and a companion measure in the House have been moving slowly, and that pace isn't expected to change when Congress comes back to town. But the administration claims to have eased things already; in discussing the number of Cuban refugees who are believed to be homosexuals, a spokesman stated that INS "no longer keeps people out just because they are homosexual." If so, that's news -- long overdue. It is ridiculous for INS inspectors to waste taxpayers time seeking out, grilling and otherwise harassing people on this issue. Sexual conduct, unless it is violent and criminal, should not be a criterion for determining who does or doesn't qualify to enter the United States.