The Supreme Court let stand yesterday a ruling that a state that makes it a crime for an adult to have sex with a consenting juvenile girl must also make it a crime if the consenting juvenile is a boy.

The 1st U.S. Circuit Court of Appeals handed down the ruling last October in overturning a 1973 conviction obtained under a New Hampshire law making so-called statutory rape a crime only when the consenting juvenile is female.

The law - since replaced by one that is gender-neutral - denied the equal protection of the laws guaranteed by the Constitution, the appellate court held. Its ruling will control in four states - Maine, Massachusetts, New Hampshire and Rhode Island - and in Puerto Rico.

In addition to New Hampshire, at least 20 other states, including Maryland, have repealed traditional statutory rape laws and substituted gender-neutral ones. Yesterday's action may lead other states to act similarly.

Chief Justice Warren E. Burger and Justice Harry A. Blackmum dissented from the Supreme Court action, saying they would have summarily reversed the 1st Circuit, notwithstanding the new statue.

The case involves Thomas E. Meloon, who in 1974 was sentenced to seven to 15 years for statutory rape of a 14-year-old girl. His lawyer said that they had "maintained a social relationship," and that the girl had had "a number of prior sexual experiences" and had had sex with Meloon "willingly, even lying about her age in order not to discourage" him.

Meloon, now 29, served 3 1/2 years of the sentence, being released on bail when the appeals court ruled in his favor.

The court took other actions: LIBEL

The justices left intact a South Carolina Supreme Court ruling that a newspaper shows "actual malice" toward a public official when it departs "from responsible standards of investigation" to print articles "on the basis of an admittedly unreliable source without further verification."

The ruling upheld a $50,000 libel award won by James P. Stevens, Horry County's state senator from 1955 to 1976, against Sun Publishing Co., owner of the Sun News in Myrtle Beach. The case involved the senator's brother, known as Tommie McLeod, who was one of seven stockholders in a controversial tidelands development, Sandy Island, and his former wife, Marlene.

In a lawsuit naming the senator as a defendant, Marlene McLeod alleged that he had pressured a magistrate into issuing unlawful warrants for her arrest. Later, in news stories about Sandy Island, reporter John Monk wrote that she had accused Stevens of an abuse of political power - which was the way Stevens himself characterized the allegation in trial testimony. The state supreme court held, however, that the Sun News "incorrectly stated (Stevens) was being sued for abuse of political power."

At another point, Monk wrote that she had told him that her former husband was "an incredible con man and business manipulator who had ruined many people's lives" and given the senator "nothing but headaches." She also said that Stevens "will do pretty much as Tom tells him. Tom uses Jems."

There was no dispute that she had said those things. The court declined, however, "to extend constitutional protection to articles containing blatantly false statements and opinions of a biased informant which imply improper conduct by a public official." Sun Publishing said the bias related to the former husband, not Stevens.

In its petition for U.S. Supreme Court review, the publisher said the allegation of "blatantly false statements" is "without support in the record." The petition also said that the decision violates the court's requirement that a public official seeking libel damages must provide "clear and convincing proof" of actual malice, that is, that a statement was made "with the knowledge that it was false or with reckless disregard of whether it was false or not." COAL MINE SAFETY

The court delayed the expected trial of a 172-count 1975 indictment accusing Consolidation Coal Co., and eight employes of consipiring to violate the Federal Coal Mine Safety and Health Act, knowingly filing false reports, and willfully violating standards set under the law to protect miners from black lung disease. The indictment derived from search warrants challenged by the Continental Oil Co. subsidiary, but upheld by the 6th U.S. Circuit Court of Appeals. The justices sent the case back for review in light of two of its recent decisions on such warrants. HIGH SEAS

The court ruled, 6 to 2, in a damage suit for wrongful death on the high seas, the victim's survivors are eligible to collect for their "pecuniary loss," but not for loss of companionship. ELECTION FINANCING

In a Minnesota case, the court let stand a ruling upholding a unique system of public financing of state elections in which a taxpayer can designate the major party he favors, but the state pays the resulting $1 donation directly to candidates nominated by the party after the income tax returns have been filed. JOAN LITTLE

The court cleared the way for the immediate return of Joan Little from New York City, where she was captured last Dec. 7, to the North Carolina state prison from which she had escaped. The court rejected a petition alleging that she will be subject to abuse and possibly will be killed. Little who is black, had been incarcerated on breaking-and-entering charges. She was acquitted of murdering a white jailer who, she said, had tried to rape her. ABUSIVE WORDS

In a case from Kettering, Ohio, the court declined, 7 to 2, to review a ruling upholding the misdemeanor conviction of Alton Sonstegard for violating a city ordinance prohibiting a person from "communicating unwarranted and grossly abusive language to anyone." In the presence of a policeman who had stopped him in predawn hours and had inquired if his companion was his wife, Sonstegard said, "sonofabitch." He was fined $100 and jailed for a day.