DEAR ABBY: We are a middle-class family with a 20-year-old son and a 16-year-old daughter. Our problem is the girl -- "Karen." She's been going with the same boy, "Mark," for a year and a half. He's 17, and they are so wrapped up in each other, it's terrible.

Karen was always a happy, outgoing girl, but now she's secretive and quiet. She used to confide in me. She doesn't go places with her girlfriends like she used to; they don't even call her anymore. Her grades have plunged this year, so she has to go to summer school to make up some credits.

I have tried to reason with her. We have restricted her to seeing Mark only once a week because they were getting too thick. (She says she "loves" him.) We have talked until we are blue in the face about doing something besides waiting for Mark to call, but she can't -- or won't -- see the light. What can we do? -- KAREN'S PARENTS

DEAR PARENTS: Your daughter has a bad case of lovesickness. She needs someone she can talk to honestly about her feelings. Unfortunately, there is now little communication between you and Karen. So, family counseling might bring you closer together.

Karen needs to see you as loving parents, which is what you are, instead of "the enemy" -- using your parental power to keep her from seeing the boy she loves. (Don't ridicule her; although she is only 16, her love is genuine.)

Ask your family doctor to recommend a family therapist. If money is tight, your YWCA may be the answer. And hats off to you for seeking help.

DEAR ABBY: I need your opinion. Two weeks ago, I gave my girlfriend an engagement ring. We have been going together for nearly two years, and it's understood that we will get married in about a year, so she wasn't exactly surprised. (I am 24 and she is 21.)

Now, the problem: The ring I gave her is a three-carat zircon -- set in white gold. It looks exactly like a diamond, but it is not nearly as expensive. I never did tell her it wasn't a diamond, and she has been showing it off at work and to her relatives. Now I'm too embarrassed to tell her the truth.

I keep thinking that one day, when I can afford it, I will replace it with a real diamond, but now I don't have the nerve to tell her the truth. What should I do? -- NO GUTS

DEAR NO GUTS: Please get the guts to tell her the truth, and the sooner the better. If she should decide to insure the ring, she would be told immediately that it is not a diamond. Your engagement would be less "rocky" if she heard it first from you.

DEAR ABBY: My mother-in-law's behavior is sometimes very puzzling to me. She will bake (or buy) some sort of pastry or dessert to take with her when invited to a get-together for a special occasion. (Birthday, anniversary, housewarming, etc.)

When the get-together is over, she always goes to the kitchen and packs up whatever is left over from her "offering" and takes it home with her. Trying to get her to leave even a portion of it is like pulling teeth!

I'm embarrassed when she does this in front of my family, or even her own children. My husband and I feel that whatever she brings to the home of another is a gift, and she has no right to take home even a part of it. -- CONFUSED IN MARIPOSA, CALIF.

DEAR CONFUSED: The leftovers should be left -- unless, of course, the host or hostess insists that the donor take them home. And by the way, when one takes a "goody" to the home of another, it would be very thoughtful to bring it on a paper plate, a box, or some other type of disposable container that need not be returned.

Abby's family recipes are included in her cookbooklet. Send a long, self-addressed envelope, plus check or money order for $3.95 ($4.50 in Canada) to: Dear Abby, Cookbooklet, P.O. Box 447, Mount Morris, Ill. 61054. (Postage is included.)

DEAR ABBY: My husband and I adopted a beautiful little boy two years ago. He was 2 years old when we got him. We waited for 2 1/2 years to get him, and when the agency called us to say we could come and get him, I was so thrilled I shared the good news with my co-workers. I worked in an office with 13 other women. In the three years that I worked there, I attended six baby showers and hosted one in my home.

Abby, I cannot tell you how heartbroken I was when no one from the office even called to ask how things were going. No shower, no gifts -- not even a card. I did, however, receive many questions about my son's birth parents. (Did I ever meet them? Were they married? How old was his mother? Does he have any sisters or brothers?)

I since have moved on to a new job because I felt so much resentment toward my co-workers. I was invited to two more baby showers before I quit.

Abby, please remind people to treat everyone's new child the same -- adopted or not. It would mean so much to the new parents. I just had to get this off my chest. Thank you for listening. -- BROKENHEARTED WITH A TERRIFIC SON

DEAR BROKENHEARTED: There is no defense for the attitude of your former co-workers, but I'm printing your letter because it carries a very important message! An adopted child deserves the same celebrated welcome as a birth child, and because there is usually so much red tape, praying and waiting, perhaps a little more.

DEAR ABBY: I am a female, working for a cable television company as a computer operator. Lately, every morning when I sign in on the computer, I find suggestive messages of a sexual nature from the man I relieve from the night shift.

One message: "Darling, I dreamed about you last night ..." He also had flowers delivered to me at work on Valentine's Day.

I am a single mother and this is starting to get out of hand. I am dating another man and have no interest in this co-worker.

Should I report him to my supervisor? Someone in the office suggested that I file sexual harassment charges. What if I do, and the guy gets canned from work and tries to hurt me or get even? -- HARASSED IN DENVER

DEAR HARASSED: Before you report him to your supervisor, tell the pushy pest that you are not interested in his flowers or his fantasies. And if he annoys you further, make good your threat.

DEAR ABBY: Please help me. My desk at work is 10 feet away from a person who whistles constantly. I get so irritated that I cannot concentrate on the task at hand. This is beginning to have a great effect on my work, and I don't like it. The problem is that this person is my boss, and I don't want to run the risk of seeming insubordinate. On the other hand, if this whistling does not stop, I will go crazy. What should I do? -- SOON-TO-BE-BONKERS

DEAR SOON: Your job can't possibly be more important to you than your sanity, so tell your boss that you can't concentrate on your work while "someone" is whistling. And if your boss continues to drive you bonkers with his/her whistling, repeat the message, unless, of course, you don't mind going bonkers.

Want your phone to ring? Get Abby's booklet, "How to Be Popular" -- for people of all ages. To order, send a long, business-size, self-addressed envelope, plus check or money order for $3.95 ($4.50 in Canada) to: Dear Abby, Popularity Booklet, P.O. Box 447, Mount Morris, Ill. 61054. (Postage is included.)

DEAR ABBY: Sex for seniors? Why not? That's right up my alley. I'm age 85, with a 71-year-old third wife, and we make love frequently. I lost my first two wives to cancer after almost 20 years with each.

I met my present wife when I was 79 and she was 64. She had been with the only man in her life -- her husband of 37 years -- so I should not have been surprised at her obvious lack of experience. Being "made love to" was new to her, as opposed to being an outlet for a man's desires. Once we got in tune with each other, we made love 10 times in nine days -- once every day, and twice on Sunday. I kid you not.

We have slowed down somewhat since I was hospitalized for a month after surgery to remove a cancerous colon. It was a month after that before we could resume our lovemaking. Notice, I said "lovemaking," not having sex. There is a difference.

By the way, I met my third wife in church. After our relationship became serious, we toured Europe together. Different names on passports don't bother hotel clerks or anyone else when two people travel as a couple. After living out of a suitcase for six weeks, we knew we'd either love each other or hate each other's guts. We married as soon as our property sales were completed.

My secret for a successful marriage? Take your time. Sex is a quick roll in the hay. Making love is like an ocean voyage; half the fun is getting there. -- OLDER BUT NOT TOO OLD

DEAR OLDER: You could write a book ... you certainly know what to do between the covers. Hooray for you!

DEAR ABBY: Could you please tell me if there is anything a man can do to color the gray hair on his chest? Are there any products on the market that can handle this problem?

The hair on my head is mostly dark brown with only a few gray hairs, but I am getting a whole flock of gray hairs on my chest. Hurry your answer, please. I don't want to look like an old guy on the beach. -- MALIBU MALE

DEAR MALIBU: Ask your barber. He should know. If he can't help you, head for the nearest store that carries beauty supplies. Its shelves will be lined with "rinses" and "tints" that will solve your dilemma.

DEAR ABBY: Please say something about the widespread use of pacifiers. They are used mostly to pacify the parents who stick it in the child's mouth to keep it quiet.

Putting a plug in a child's mouth, whether it's a genuine plug or a bottle (when used as a pacifier), causes the child to associate comfort and relief of stress with oral satisfaction, which may lead later in life to smoking, overeating or alcoholism.

The child's instinct to suck is usually satisfied by breast or bottle when the child is hungry.

And don't you hate to see a child who is old enough to walk still sucking on a bottle? -- BARBARA SNADER, CHILD DEVELOPMENT SPECIALIST

DEAR BARBARA: Yes. But the people who most love to see a child who is old enough to walk still sucking on a bottle are the orthodontists.

By popular request, Abby shares more of her favorite prize-winning, easy-to-prepare recipes. To order, send a long, business-size, self-addressed envelope, plus check or money order for $3.95 ($4.50 in Canada) to: More Favorite Recipes by Dear Abby, P.O. Box 447, Mount Morris, Ill. 61054. (Postage is included.)