She's married. So's he. Yet sparks fly. Can close friends become too close?

I am having lunch with a smart and funny man who makes me feel young and unencumbered, even though I am neither (I'm 56, and my body has weathered the birth of four sons). He is tall, with a slight bulge at the belly and wispy white hair. I see instead a taut 24-year-old with wavy blond hair and a boyish grin. I dated him briefly after college, but never slept with him.

See also: 4 ways to heat up your sex life.

Flirty friendships can enhance all the relationships in your life, including your marriage. — Photo by Jesse Rieser

For more than three decades, the two of us have enjoyed a friendship that is flirty and unwavering. "You look the same as the day I met you," he often says, recalling a Friday-night party in 1977 to which I wore a velvet maxiskirt and lizard-skin cowboy boots. He is lying, but — swelled by sweet and addictive affirmations not readily available at home, where my husband and I share teenagers and bills — I believe him. So during one lunch every month or so, my old friend and I remember together who we were: flat-bellied singles drinking Labatts on Oak Street Beach until dawn shimmered over Lake Michigan. We have a crackle and connection that is unrivaled by any of our other relationships, anchored by roots and an unrequited attraction.

We are also both in lengthy marriages with spouses who are flexible, confident and aware of this simple truth: Every time we see each other, we come home in spunkier moods.

"Old friendships bear witness to your life in its entirety," says Chicago psychotherapist Carol Moss, coauthor of A Woman's Search for Inner Peace. "And if you are a heterosexual woman, having this friend be a man makes those memories even more intense. From the work that I do, I really think that's all we want in our relationships — to be truly known."

The ancient art of flirting, defined as "behaving amorously without serious intent," is easy to do, sexy without the sex, and enormous fun. In old boyfriends we find our history holders; in new men friends we get the endorphin rush of a first date. Gone is the cultural stereotype of male bosses attended by flirty secretaries in décolletage. The modern reality is that female executives, many in their own plunging necklines, now hold growing numbers of the highest-earning jobs in the United States, giving them equal opportunities to meet intriguing members of the opposite sex, in boardrooms and all over Facebook.

Next: Navigate your flirty friendship without crossing the line. >>