It may be love at first sight, but Baltimore Catholics must wait six months if they want to be married by a priest, under a new policy announced yesterday by Archbishop William D. Borders.

While they are waiting, they must participate in formal marriage preparation courses that have been certified by the Baltimore Archdiocese.

Under the policy, priests are expecteed to move even more slowly in marryng persons under 21, and watch out for couples under pressure -- either from parents or because of a premarital pregnancy -- to marry.

Only one other diocese of the Roman Catholic church in this country, Phoenix, Ariz., requires this long waiting period. Locally, the Diocese of Arlington requires a three-month waiting period. The Washington Archdiocese guideline "recommends" that couples wait at least three months, but this is not a hard and fast requirement, a spokesman said.

The Baltimore Archdiocese's new policy, which will be mandatory after Jan. 1, but may be implemented immediately by individual priests, is designed "to give couples a time to think through their readiness to undertake the commitment" involved in marriage, the archbishop said yesterday.

In the Roman Catholic church, marriage is a sacrament and must be performed in the church by a priest or a deacon. But though marriage is considered indissoluble, the divorce rate among American Catholics approximates that of the population at large.

The archdiocese's tough new policy reflects the church's concern over this divorce rate and efforts to stem it through better preparation for marriage, the archbishop said.

The new Baltimore policy is spelled out in an attractively illustrated 12-page booklet, entitled "Marriage in the Lord," addressed to prospective brides and grooms.

The booklet spells out certain circumstances that might "necessitate more extended consultation" and possibly a refusal by a priest to marry a couple.

These include cases in which either or both partners is under 21 "or shows lack of the maturity demanded by Christian marriage;" refusal to take part in the marriage preparation sessions "in good faith;" situations of "substantial and undue pressure to marry from external sources, e.g. social, religious, parental pressure or premarital pregnancy," and a "strong written objection" by a parent or guardian.

Pressed on the question of pregnant brides, Borders said: "Pregnancy itself is not sufficient reason for a marriage. You don't get married just because the child involved would be born out of wedlock."

He stressed that the church should "give guidance, counseling and support in such cases, "but not encourage even a civil ceremony if the couple does not appear ready for the responsibilities of marriage.

Another circumstance that could throw an ecclesiastical roadblock into marriage plans would be "if you refuse to practice your faith or have no intention of returning to it, or if you have come to the church merely to satisfy your parents or to seek the atmosphere of a church wedding," the policy says.

If the priest, for any of these reasons, is "unable in conscience" to go through with the marriage, he must inform both the couple and the archbishop in writing.

The couple may appeal, and they may shop for a more sympathetic priest, but he would be barred from proceeding with the wedding plans until the archbishop made a decision on the appeal.

Although the Roman Catholic Church is less than enthusiastic about interfaith marriages, the Baltimore statement acknowledges that they exist and makes hospitable provisions for accommodating them.

"The minister or rabbi should be made welcome to participate in the wedding ceremony when it is held in the Catholic church, and the priest or deacon will normally accept the invitation to participate in the wedding when it is held in the church of the other party," the booklet states.

But marrying a Presbyterian, say, or a Jew, is no way to short-circuit the waiting period or the counseling. Both wll be required in Baltimore before a priest will participate or the marriage will be recognized by the church.