By a one-vote margin, the Supreme Court yesterday struck down a New york State law that allows an unwed mother-but not the unwed father-to block adoption of their child by with-holding consent.

With another 5-to-4 ruling, however,the justices upheld a Georgia law that also draws a sex-based distinction between the parents of a child born out of wedlock.If the child is killed in an accident, the law permits the mother but not the father, unless he has legitimated the child, to sue for damages.

The decisions could have important implications for some of the 15.5 percent of all children who, as of 1977, had unmarried parents. But Justice John Paul Stevens, in a dissenting opinion in the New York case, asserted that the ruling won't apply retroactively to "millions" of adoption decrees entered without the natural father's consent.

Justice Lewis F. Powell Jr. played a crucial role in both decisions, writing the majority opinion in the New York case and providing the swing vote in the Georgia case.

For him, the key issue was whether the gender-based distinction in either law violated the equal protection of the laws guaranteed by the Constitution.

He found a violation in the New York law because it "does not bear a substantial relation to the state's interest in providing adoptive homes for its illegitimate children."

But, Powell wrote, the distinction in the Georgia law doesn't deny equal protection "inasmuch as it is substantially related to the state's objective of avoiding difficult problems in proving paternity after the death of an illegitimate child."

The adoption case came from New York City, where Abdiel Caban and Maria Mohammed lived together, and represented themselves as husband and wife, for nearly 5 1/2 years. They had a son in 1969 and a daughter in 1971. Abdiel contributed to their support through 1973, when Mohammed left, took up residence with another man, and shortly married him.

Even so, Caban for the next two years was able to see the children often. Finally, he sought to take custody of them. This led to litigation in which adoption of the children was sought both by Caban, who by this time had married, and Mohammed and her husband.

In the lower courts, Mohammed won, partly because she would not consent to adoption by Caban-and he was prevented by the law from blocking her.

In the opinion for the court, Justice Powell wrote:

"The effect of New York's classification is to discriminate against unwed fathers even when their identity is known and they have manifested a significant paternal interest in the child.

"The facts of this case illustrate the harshness of classifying unwed fathers as being invariably less qualified and entitled than mothers to exercise a concerned judgment as to the fate of their children."

Justice Stevens, whose dissent was signed by Chief Justice Warren E. Burger and Justice William H. Rehnquist, said that after conception, for which a man and woman bear equal responsibility, "the differences between male and female have an important impact on the child's destiny."

Stevens pointed out, for example, that the woman carries the child and along has the constitutional right to decide whether to bear it, sometimes is the only one who knows who sired it, and, by marrying a different partner during pregnancy, can make the child legitimate when born.

Such differences justify "some" disparate treatment of the mother and father in the adoption process, he said. "Most particularly, these differences justify a rule that gives the mother of the newborn infant the exclusive right to consent to its adoption."

In addition, Stevens said, if both natural parents get a veto, as required by the majority, "Neither may adopt and the children will remain illegitimate."

He also said that in the commoner adoption situations, the safest assumption is that "the mother will be the more, and often the only, responsible parent, and that a paternal consent requirement will constitute a hindrance to the adoption process."

Justice Potter Stewart wrote a separate dissent.

Stewart wrote the opinion for a plurality of the court in the Georgia case, which arose from the death, in an auto accident, of Cassandria Moreen and her child. The father, Curtis Parham, had contributed to the child's support and visited him regularly, but never took the opportunity afforded by state law to legitimate him.

Because Parham failed to remove what Stewart termed "the stigma of bastardy," he was unable to sue for the child's wrongful death. By contrast, Stewart said, the father in the adoption case couldn't change his children's status.

In the dissenting opinion, Justice Byron R. White said it is "untenable" to assert that a state interest justifies the sex discrimination under which the unwed mother, had she survived and not legitimated her child, could have sued. Justices William J. Brennan Jr., Thurgood Marshall and Harry A. Blackmun signed White's opinion.

The decision affirmed the Georgia Supreme Court, which suggested that most unmarried fathers would suffer "no real loss" from a child's death. "Incredible," said White.