It's easy to see why Marlo Thomas has fallen for Phil Donahue. He's liberated but quaint a celebrity yet a home-body, ambitious but kindly, intelligent decent, cute. For the same reasons Donahue has fallen for Thomas.

The result is an old-fashioned drugstore romance, circa 1948. a two-straw affair with lots of starry-eyed gazing, hand-holding, stolen glances, secret smiles. In one sense, though, it's a modern match, each sweet rendezvous beginning and ending in an airport.

Donahue, the Emmy-winning TV talk show host being touted as a possible "Today" show achor, is based in Chicago. Thomas, an Emmy winner herself, lives in Los Angeles. They're forever flying to one another's arms. Last weekend they met in Washington, she arriving from New York, where she was making a children's record, he coming here to be a panelist for a discussion on raising children.

As a bachelor father of four teenage sons (divorced four years ago, he has a daughter who lives with his former wife), Donahue 42, told about 100 members of a private school parents' group asembled the other evening at Landon School how he mixes his ear pool and upbean career. Constantly alteative. Thomas, 40, looked on from the back row. He seemed easy to like.

He has a boyish face, an authoritative-looking head of gray hair, and quick wit, all familiar and lovable attributes to his increasing legion of fans around the country. His hour-long, five-days-a-week syndicated "Donahue" show goes out to no fewer than 134 stations, including Channel 9, which shows it at 9 a.m.

At first, Donahue looks like a copy of Mery Griffin and Mike Douglas: actually, he's more an Irish Don Meredith, similar not only in appearance but sense of humor. Donahue also has gained a reputation as a tough interviewer and serious commentator, in the style of Tom Snyder. His show is a departure from the usual variety/talk show fluff - he operates from his audiences, skillfully weaving its questions with his own. He devotes entire hours to his topics, which he tries to make current and controversial. Last week he covered the censorship of textbooks, UFOs, Billy Carter, male sexuality, and the circumstances surrounding the death of a Chicago youth after a police shooting.

Occasionally, he takes his show on the road; he's done a week of shows from inside the Ohio State penitentiary and a 2-part interview in Germany with Albert Speer.

But he seems most comfortable with a small audience, either in the studio or as the other night before the Parents Council of Washington. He elicits others' remarks easily because he comes across as one of them - "I'm here to learn from you" - and because he can be positively charming.

"I have already noticed your socks," he says to a young man in one of the front rows. And to the rest of the group at Landon: "And if you invited me into your homes I would notice your floors." So goes the education of a bachelor father. Smiles from the audience; a smile from Thomas.

"The biggest problem," he continues "is finding help. You get a housekeeper and in three days you have a surrogate mother. You know; the kids should do this, they should do that, their room is dirty. I'll say, Mrs. Such and Such, don't go up there. Well, I know it's up there, she'll say. I say, Okay, I'll talk to him. With a single housekeeper, it's almost like I had another child.

"Now I've got a live-in couple. They're Yugoslavian. She speaks very good English. I do charades with him."

Of course, he's fortunate enough to be able to afford such an arrangement and is quick to admit this: "I make a pretty good dollar - we may as well get that straight."

Still, there are plenty of pressures as a bachelor father.

"My youngest takes guitar lessons every Thursday night. He goes with another kid. I haven't been there the last couple of weeks. I know the other kid's mother is mad with me. She's had to do the driving. But I'm determined to drive next Thursday and the next Thursday - both ways."

Donahue says he is "playing catchup," just learning the ways of children. He also says he was late in understanding women: "I was a product of the '50s . . . raised to get out and become a VIP in the business world . . . I was exhibit A at the feminists' convention - Well, here's what we want to avoid."

Then came his enlightenment, except, "It's a shame divorce had to be the vehicle for the raising of my consciousness." He says he grew up thinking "that a girl was an occasion of sin," and that it never occured to him "to ask a date what she thought of Eisenhower." Now he's a member of NOW.

At least once during the panel discussion, Donahue peers around heads in the audience, looking for Thomas. She's sitting forward, as if hanging on every word. Still, they're in the same room, which has not been the case all that often since they met a year ago when she was a guest on his show.

Donahue, smiling, tells the audience, "I have this dream that I'm enchanting the world while my kids are at home smoking grass . . . I go on a week's trip and take two of them along. It halves my worry." Worry is a part of his upbringing that he hasn't yet overcome.

"The Irish are pretty much gloom and doom," he says. "You know, they make a plane and the Irish say, 'When will it crash?'" He was raised in Cleveland, attended Notre Dame, got into radio, and, upon graduation, went nowhere in a hurry. He was a bank teller for a while, then landed a TV job in Dayton, where he polished his skills as an interrogator - "I was the guy in the street with the microphone and police chief" - and began his current show. Sensing that Dayton "is not the show business capital of the world," he moved the show to Chicago, where in the last two years his career has taken off.

These days he is rumored to be headed for New York when his contract is up in April, 1979.He is coy when it comes to talking about his ambitions, saying things like "Just recently I feel comfortable with my career" and "A lot of people go through life thingking what they're going to be doing next week" and "In a network situation, one guy can end your career when he's shaving."

One gets the feeling that's only part of his feelings, just as when he discusses the possibility of a second marriage. "I don't think I'll go to the grave single," he says. "Like most divorce people, I'm a little gun-shy."

Thomas isn't saying either, of course, but she mkes no secret that she finds it easy to like him. "Hey, you were great. I dug that," she tells him after the panel discussion.

"C'mon," he says, "I want to have words with you," and they talk, almost nose to nose, off by themselves, for a minute.

Later, she says of him, "He's so curious, inquisitive. He enjoys himself." She says he's not always "turned on" as he seems on the air, or as a panelist in Bethesda. She says he'd be a "machine" if he were, and she goes on to say he's not that at all.

What he seems to be is the kind of man a liberated woman can lean on. Descending some icy steps, she calls out, "Oh, Phil, where are you? I think I'm going to fall."

As in all fine romances, he's right there.