WHY IS SOMETHING that seems so clear, so simple, so utterly fair as the Equal Rights Amendment causing so much trouble? How on earth, in this day and age, as we say, can anyone be against amending the constitution to read:

"Equality of rights under the law shall not be denied or abridged by the United States or any state on account of sex. The Congress shall have the power to enforce, by appropriate legislation, the provisions of this article."

What is the Equal Rights Amendment and what would it really do?

Phyllis Schlafly, the founder of the Stop ERA movement who is currently starring in the GOP convention, believes the ERA is part of a plot by the women's liberation movement to create a unisex society. "ERA will make federal and state laws treat men and women as equals," she contends. To her, this means it will force women out of the house and into combat. Put another way, it will jeopardize the Great American Deal: women no longer will have a legal right to be supported by their husbands in return for nurturing children and making a home. To Schlafly and her followers, ERA strikes at the heart of the American family.

"ERA would prohibit any law that says a husband must support his wife. Wives would be big losers," she says. ". . . I think it's the centerpiece of the goal of the antifamily forces because with one amendment it would take away traditional rights of wives; it would give new rights to homosexuals; it would give the constitutional right to abortion funding and it would give the federal government power over family law, which today is in the hands of the states."

Anti-ERA activists also argue that laws guaranteeing equality in such important areas as pay are already on the books and the ERA, far from helping women get economic equality, will only cause trouble. Women such as Shirley Spellerberg, a leading ERA opponent in Florida, calls for enforcement of existing legislation, not ERA. "I'm for women and I'm for women's rights," she argues, "which is why I'm so unalterably opposed to the ERA, which is going to strip women of their privileges under the law."

Will it really? Harvard law professor Laurence H. Tribe, one of the foremost constitutional scholars in the country, takes a more sanguine view.In his book, "American Constitutional Law," he writes:

"Although it is difficult to forecast the precise boundaries courts would delineate in interpreting the reach of the ERA, it is clear that many of the concerns expressed by opponents are exaggerated, and that much opposition, both male and female, derives from an unwillingness to accept those new responsibilities which may accompany the ERA's new rights.

"Included in the litany most frequently reppeated by detractors of the ERA are fears that women will lose the right to their husband's support and the benefits of protective legislation in the labor area and others. But . . . much of the 'protection' currently offered simply impedes the voluntary pursuit by women of roles for which they are eminently qualified."

Judith Lichtman, executive director of the Woman's Legal Defense Fund, argues that many of the so-called homemakers' rights the anti-ERA forces feel are threatened don't exist in a legal sense in the first place. "Married women have no contractual right to support from their husbands," she says.

What ERA would do, she says, is force states to take sex out of the state codes and change a whole range of anachronistic laws. "In many states, when a husband and wife are married they become one and the one is usually the husband," she says. In such states, domicile is determined by where the husband is. A husband can desert his wife in one state, move to another and file for divorce in the new state on the ground that his wife deserted him. Why? Because your legal residence is where your husband is, like you're a piece of property."

The Women's Legal Defense Fund has sued Maryland and Virginia on behalf of women who were paying in-state tuition at state universities but were forced to pay nonresident rates when they married men whose legal residences were out-of-state. The women were both lifelong residents of their states. The Maryland case still is in the courts, but the Virginia case was won when the attorney general decided that, for tuition purposes only, the married woman could retain her legal residence. But, says Lichtman, "she could no longer vote in the state of Virginia and could no longer serve on a jury in Virginia. It's that kind of law that the ERA would get at. You can't legislate legal residence based on sex."

To Phyllis Schlafly, the most important issue about ERA is the draft. Attempts to draft women were defeated in Congress but had the ERA been in effect, she says, "women would have had to register and be drafted just like men . . . We're so fortunate we don't have ERA."

Lichtman's answer to that is President Carter tried to draft women without the Equal Rights Amendment. "If Carter has shown us anything it's that he doesn't need the Equal Rights Amendment to draft women. It's just totally irrelevant to questions of the draft."

As for unisex toilets and integrated prisons, she says that courts will have to balance the First Amendment right to privacy with the Equal Rights Amendment right not to be discriminated against on the basis of sex. "The courts are very expert in evaluating competing constitutional rights. There may be times when the right to privacy prevails."

The Supreme Court came within one vote in 1972 of making a ruling that would have had the effect of enacting the Equal Rights Amendment. It stopped short, in part because two justices believed that ERA should become the law of the land by making its way through the state legislatures.

That has not happened and the court in subsequent years never has ruled that sex discrimination, like a race discrimination, is unconstitutional. While some people argue that the ERA, in view of all the laws already on the books, would simply be a symbol, it would clearly be much more than that. It would, for example, have made it much more difficult for the Supreme Court to have upheld veterans' preference legislation -- which the court acknowledged discriminates against women by giving veterans first crack at government jobs.

Lichtman says a major reason for the Equal Rights Amendment is that there needs to be a "clear and concise statement of the evil of sex discrimination and the seriousness with which the nation and therefore the court will treat such acts."

That, after all, really doesn't seem like such a bad idea.