“Blue Bloods” star Tom Selleck has made women swoon since his days on “Magnum, P.I.,” but the ’80s heart- throb is a one-woman man who’s discovered the secret of how to have a happy showbiz marriage! 

The still-handsome 69-year-old ac tor and his second wife, British-born beauty Jillie Mack, 56, have been enjoy ing marital bliss since 1987 – because they made a pact to never fight. “We decided long ago that fighting was a waste of time,” a pal quotes Tom as saying.

“So we said that if we had a difference of opinion about something, we would sit down over a cup of tea or a meal and talk it through. “Jillie has a great sense of humor, so our talks often end with both of us laughing, and as they always say, laughter is the best medicine.”

The lovebirds live with their 25-year-old daughter, Hannah – a professional horse show jumper – on a 65-acre avocado ranch in Ventura County, Calif. And Jillie appreciates what a catch she has in her husband – he’s both handsome AND a hopeless romantic.

“Tom dotes on Jillie,” said a source. “He gets up early every morning, makes her a cup of tea and takes it to her before she gets out of bed.”

He also leaves little love poems he’s written around the house for her to find while he’s in New York every other week filming “Blue Bloods,” adds the source.

“And he’s constantly texting her sexy messages. He’s also on a first name basis with the local florist because he’s always sending her flowers.”

Another reason the marriage is so strong is be cause Tom – who was wed to model Jacquelyn Ray from 1971 to 1982 – and Jillie understand the importance of giving each other breathing room.

“Jillie and Tom agree that although it’s nice to have some shared interests, people need to have their own individual interests, even if it means doing things on their own,” the source added.

“They say when they come back together again after time away, the sparks fly – so it works out great.”