Q.I am really worried over the situation my 19-year-old son got himself into when he was in high school.

At that time he began a relationship with a dropout he met at a party, and the relationship continues even though he has graduated from school and joined the Army. She, however, is still 16 and living with her mother.

I know the girl's mother lets them have sex at her house because she would call me to ask if my son could sleep over when he was still in school.

When I talked to her about our values and how early sex was wrong, she said that "the kids" were going to do it anyway, so they might as well be comfortable.

I have tried to tell our son that the girl isn't old enough to have a sexual relationship and that his behavior is not only illegal, it is morally wrong.

He says that it is legal, since their ages are within four years of each other, and that if her mother doesn't mind, why should we?

What, if anything, can I do?

A.You can't undo the undoable.

The joy of sex keeps the human race going, and once tried, it can be addictive, especially in adolescence, when passions run high and good judgment is elusive. This can lead teenagers to think that their relationship is the deepest, purest, finest love there ever was, even if it looks foolish, risky and irrational to most parents.

It's time to discuss sex with your son, but with complete respect. If he's old enough to enlist, he's old enough for you to talk with him, one adult to another.

Tell your son that you and your husband want to have a full and friendly -- and final -- talk with him and his girlfriend about their sexual relationship.

She will probably join him if you promise not to be angry or judgmental and they will probably listen if you keep your word about that.

Talk first about the legal aspect, telling your son that he seems to be right. A sexual relationship is usually legal if the girl is 16, although the law varies from state to state.

Don't talk about moral issues, however. Simply tell your son and his girlfriend that they may not share his bed at home, even though they sleep together at her mother's house, and that you hope they will be discreet about their sexual relationship, to protect her reputation and your good name. This will seem old-fashioned to them, but you have to be true to yourself no matter what anyone else says or does.

What they really need is a reality check about sex, starting with birth control. Most teenagers don't know that it must be used with care, since abstinence is the only surefire way to prevent pregnancy.

Tell your son that he should use a condom, if either of them has other sexual partners, because it is the only contraceptive that can keep them from picking up a sexually transmitted disease, including AIDS, and passing it on.

If neither has other sexual partners, his girlfriend may prefer birth control pills, but she should know that they may not be as effective as they should be if she is taking certain antibiotics, such as penicillin, or if she is on an anticonvulsant or Dilantin. Or if she occasionally forgets to take a pill.

Then ask them what they would do if she got pregnant. Would they marry? Would she get an abortion? Would they give the baby up for adoption? Or would she rear the baby on her own? If so, does your son know that he will have to pay child support every month for 18 years?

With luck and care, your son's girlfriend won't get pregnant but if they're like most teenagers, one of them will get bored with the relationship or fall for someone else, and it will break the other's heart. Although you shouldn't dwell on that, ask them to be honest about their feelings for each other, so a breakup won't be a painful surprise.

But whatever you do, let them know that as long as their relationship lasts -- a month or a lifetime -- you will be there for them. And mean what you say.

Once the Big Talk is finished, quit thinking about their sex life and support their friendship instead. Invite the girl to dinner with your son occasionally, give her a present for her birthday and try to establish a connection with her.

But don't expect them to stop having sex or for your son to turn down those offers to stay overnight at her house. Turtles would fly first.

Questions? Send them to advice@margueritekelly.com or to Box 15310, Washington, D.C. 20003.