Q: I have a shy, 17-year-old grandson who suddenly began dating and has no time for his friends.

His parents are disturbed because the girl has no restrictions and because these teenagers work in the same place after school. They feel too much togetherness is a bad thing, even though the boy is keeping up with his studies.

How should the parents handle this situation?

A: A few dates and a lot of togetherness at 17 often turns into a heavy relationship, with all the joy and sorrow that can entail. There is perhaps no love as sweet or intense as a teenage romance, nor as scary for grown-ups.

While adults know that young love is full of pitfalls and pain--and sometimes pregnancies--it's a miracle to smitten teenagers. This prince and princess think they'll live happily ever after, if they're thinking at all.

In times like this, the adults have to do the thinking for them.

Unless their son's girl is very bad news, they should treat his relationship with respect. At least the young people are learning to give each other some of the generosity, patience, empathy and love they've received from their families. They may not always do it well, but they are learning to build a relationship.

The lack of restrictions for the girl is unfortunate, however, because restrictions make sex inconvenient, although they don't, of course, turn a sexually active young couple into a celibate one. Sex is much too addictive for that.

This leaves the boy's parents to set restrictions that will help both teenagers. For best results, their son should stay home on school nights, be in at a decent hour on the weekend, and either maintain his grades or quit his after-school job--a rule that might make him study harder and stay off the phone a little more.

The parents must also insist, firmly and flatly, that he and the girl can't be alone at her house--or his--and they can't stay in the bedroom, even if an adult is downstairs. According to one study, most teenage sex first occurs at home--and in their parents' bed.

The girl would be more likely to live by their rules if the parents make her feel welcome. They should let their son have occasional pizza parties for a few old buddies and their dates; invite his girl to a family dinner every week or two and talk to her as a young adult.

This openness should also make it easier for the parents to initiate a very personal, very necessary, after-dinner talk about sex.

The parents need to tell them that 3 million teens get a sexually transmitted disease every year and 905,000 girls get pregnant, according to the Alan Guttmacher Institute.

They also should tell them that the two main methods of birth control--the pill and the condom--have significant failure rates. For the pill, this rate is 7 percent to 8 percent--largely because the women forgot to take it sometimes or because they were also taking an antibiotic or had an infection--and that the failure rate for condoms is about 15 percent, again mostly because they were used improperly.

And then these teens should be asked what plans they've made if they decide to have sex and the contraception fails. Would they keep the baby? Give up college? Expect the grandparents to rear the child? Put the baby up for adoption? Choose abortion? And how would they feel about each of these choices?

It's much more sobering for teenagers to answer these questions than to get a lecture--and it's even more sobering for them to hear each other's answers. The more you can get teenagers to discuss consequences--and values--the more sensible their choices will be.

Please send your questions to Box 15310, Washington, D.C. 20003 or to margukelly@aol.com.