Sex ... read on, because that is what this is about. It is about gender and sociology, too, but come on – it all comes back to sex.

Actually, this is about the absence of sex and how that void somehow is filled with something no one can quite put their finger on when talking about platonic friendships between women and men. And yes, those friendships are possible. As long as there is no sex.

The phrases “like a brother” and “like a sister” come up often in these kinds of discussions. Jennifer Clapp and Kurt Pennypacker could hardly be any closer. They met three years ago when Pennypacker invited a co-worker out to happy hour and she showed up with her friend Jennifer. Soon they and other friends were hanging out in one big happy group. But Clapp and Pennypacker clicked in a special way.

They had the same sense of humor, and that intangible thing, whatever it is, that draws friends together. When their apartment leases ran out last year, Clapp proposed that they pool their money, find a sweet pad and move in together.

Since doing that they've learned the rules – don't speak to her in the morning unless she speaks to you first, stay out of the kitchen when he's cooking – and life at home hardly could be better.

“We kind of looked up after a year and I was like, ‘I can't believe I live with you,'” says Clapp, 31. “It's just so odd.”

But it works. And it has strengthened their friendship. As roommates, Clapp and Pennypacker have more opportunities to rely on each other, to be there for each other when things aren't going so great. Clapp especially appreciates that her roomie also is willing to not be there at times.

In the literal sense, that might mean moving out for four days, as Pennypacker did to accommodate Clapp's visiting sister and her family. In the figurative sense, not being there usually means knowing when to zip it.

“She'll come home from a bad day, and I'll be like, ‘Do you need a hug?' and she'll be like, ‘No! Leave me alone!'”

Pennypacker has learned to back away slowly in those situations.

“With guys you can say, ‘I don't want to talk right now,' and they're not going to push it,” Clapp says. “Girls tend to be like, ‘Are you sure. ... ?' Kurt is very aware of my space.”

And speaking of personal space, neither party has ever had any interest in crossing the line to romance, though they have dabbled in dating each other's friends.

“I can't really walk around in my underwear at home,” says the 28-year-old Pennypacker. “That's about the only drawback I can think of.”

They both admit, that each of them possesses a bit of the opposite gender's energy. “Kurt's like the nicest guy I've ever met,” she says.

“I cook and clean, but she also watches baseball,” he says.

Got it.

Stacy Grobe and Glenn Chernyak are platonic friends who met as students at the University of Arizona in the fall of 1996.

“Our friends had a date party and we went together,” says Grobe, 28. “Nothing ever happened. And we've been friends since then.”

Chernyak says there were clear indications that night, a month into their friendship, that romance probably was not in the cards – specifically because they both went home with different people. They laughed about it the next morning on the phone, and haven't been able to shake each other since.

Both rabid Arizona Wildcat basketball fans, they've taken road trips together, sharing the same hotel room and sometimes, admittedly, overindulging at the bar. And still, no spark.

“There's like an invisible wall, or a force field,” says Chernyak.

There was a time earlier this year when they got on each other's nerves so badly that they didn't talk for two months. Something about a road trip to watch basketball. Too much time together, not enough space.

“After that trip.” Grobe says, “we definitely know our limitations.”

Maybe that is the secret to the female-male friendship – putting a little breathing room in the middle, like any good friends should do, regardless of their gender.

One could also do worse than to heed the advice of Pennypacker, the platonic cohabitator and male presence in the Clappypacker apartment: Keep your pants on.

© 2006, Chicago Tribune.

Distributed by McClatchy-Tribune Information Services.