The Virginia House, after ensuring that couples "necking" in cars won't be inconvenienced and that "safety belt squads" won't harass motorists, gave preliminary approval today to a weakened mandatory seat belt bill.

Despite protests from the bill's sponsor, the House passed an amendment that would prohibit police from stopping motorists just to check whether seat belts were being used. It also required that motorists be charged with some other traffic offense before they could be charged with violating the seat belt law.

"It doesn't gut it, but it certainly does weaken it," said Del. J. Samuel Glasscock (D-Suffolk), sponsor of the measure, which passed by voice vote and is expected to receive final approval Tuesday. Glasscock indicated he hoped to remove some of the limitations when the bill is considered by the Senate.

Under the bill, drivers and front seat passengers in vehicles, except taxicabs, would be required to use seat belts. The state already has a law that requires children 4 years old and under to be restrained in safety seats or straps in either the front or back seats.

Glasscock, who tried for two hours to fend off attacks on his bill by opponents, persuaded the house to reject several amendments that would have further weakened the bill, including one to remove the $25 penalty and another to send the bill back to a hostile committee.

"We're talking about a very important issue," an exasperated Glasscock chided at one point.

Del. Theodore V. Morrison Jr. (D-Newport News), a defense lawyer, amused legislators when he proposed one amendment that he said would make certain police could cite only motorists who actually were driving cars.

Otherwise, Morrison said, a romantic couple "necking" somewhere in a parked car on a darkened road could be cited. "I'm sure you remember necking," Morrison deadpanned. "Necking . . . is pretty difficult to do, especially if you're strapped in."

Several delegates worried that police might be overzealous in checking seat belt use and said it could be used as a pretext to stop motorists when no other "probable cause" existed.

Del. Clinton Miller (R-Shenandoah), sponsor of the amendment to limit when motorists could be cited, said he was worried about "safety belt squads" roaming the streets.

The House approved a similar seat belt bill last year, 52 to 48, but the measure was killed in the Senate Transportation Commitee on a 9-to-6 vote.

The seat belt bill, which supporters say will save about 200 lives a year and thousands of injuries, is backed by a coalition of health care, emergency rescue and other groups.

The District of Columbia's mandatory seat belt law went into effect in December, and a similar bill is under consideration in the Maryland legislature.

The federal Department of Transportation has said that states with two-thirds of the nation's population must enact mandatory seat belt laws by 1990 or the auto industry will have to install alternative safety devices.