The American Civil Liberties Union yesterday files suit in U.S. District Court here to stop President Carter's plans to renew draft registration on July 21, charging that it is unconstiutional because it applies solely to men.

Although the lawsuit was brought on behalf of 16 men as a sex discrimination case, ACLU officials made it clear yesterday that their principal target is the draft, and that they would seek other ways to block registration plans if this effort failed.

The House gave final approval Wednesday to a bill that would provide $13.3 million needed to register all 19-and 20-year-old men this summer. Selective Service Director Bernard Rostker said Carter is expected to renew draft registration officially on Tuesday.

All 20-year-old men will be required to register the week of July 21, at Post Offices nationwide, and 19-year-old men will be required to sign up the following week, Rostker said.

ACLU lawyer Isabelle Katz Pinzler said yesterday that the ACLU will seek an expedited hearing schedule in the court so that the merits of the sex discrimination argument can be addressed before July 21. The case has been assigned to Judge Gerhard Gesell. Pinzler is director of the ACLU's Women's Rights Project.

Specifically, the ACLU wants the court to declare that the section of the Military Selective Service Act, which requires men -- but not women -- to appear for draft registration, is unconstitutional. If the law is then sent back to Congress, the ACLU theorizes that registration plans will be abandoned because lawmakers will not agree to such a bill that would include women.

The lawsuit contends that the law fails to come up to constitutional standards because the government cannot show that the exclusion of women from registration is "substantially related to any important governmental objective."

In particular, the lawsuit notes that the president, the Department of Defense and the director of the Selective Service System all recommend that women be included in registration.

ACLU lawyer Pinzler said yesterday that she expected the case against draft registration to find support in U.S. Supreme Court decisions which have found since the early 1970s that various governmental actions are illegal because they discriminate on the basis of sex. Other lawyers said yesterday, however, that the draft registration challenge is more difficult because it deals with questions involving the military.

In that area, these lawyers said, the courts have afforded the government broad discretion in its decisionmaking.

Before even reaching the constitutional questions, lawyers said yesterday, the government could argue that the men have no grounds to come to court because the case only involves registration. An actual draft might amount to an injury that could be remedied by the court, these lawyers said, but not simply a registration.

While the ACLU lawsuit makes no specific mention of women's roles in combat situations, it does note that with appropriate training women are able to serve "in virtually all' military specialities. Therefore, the lawsuit contends, it is constitutionally unjustifiable to exlude women totally from draft registration. Currently, women are precluded by both law and government policy from combat service.

There was some speculation yesterday that the government could argue that since the purpose of registration and draft is to gather and prepare forces for combat, women would thus be unable to participate even if called. It would then follow, lawyers said, that the government would not be required constitutionally to have women register for the draft.

The ACLU lawsuit indirectly supports extensive arguments that women could fulfill a wide range of noncombat military jobs if they were included in the draft. Lawyers theorized yesterday, however, that even that argument in favor of equal treatment for men and women would face tough competition when confronted with arguments centered on congressional decisionmaking directed at military security.