A long time ago, a friend of mine told me he was certain that he loved his new girlfriend, totally and utterly and helplessly.

I asked him how he could be so sure. And I’ll never forget his response.

“Because when I’m, well, you know…” he said, pausing, smirking. “I think about her.”

I’ve never quite been able to shake this revelation. Putting aside the fact that a close friend of mine couldn’t bring himself to name an act that has any number of crude and hilariously acceptable names, I wondered — and still do wonder — what images people conjure when they’re in need of sexual inspiration?

Do bored housewives really think of Brad Pitt? Or do they think of Pete, the young stud in the mailroom? Or maybe Dave, who’s married to their best friend?

And the guys — do they remember the details of Bella Donna’s latest hardcore video raunch-fest? Or do they think of Cindy — sinful Cindy — recalling that fling back in college? Or maybe Trish, the one who got away?

One friend of mine, a young writer and teacher from Chicago, said his porn habit and regular girlfriend means he hasn’t called on his imagination for some time. But he did confess that when he was 13, he secretly believed that if he masturbated to the thought of a girl he really liked, it increased his chances of making that imagined moment a reality.

On the more normal side of things, according to a female friend of mine (a young professional, who, like all sources in this story shall remain nameless) any male cast member from “Lost” will work just fine. (And she doesn’t expect it will lead to a tryst with them anytime soon.)

“I like Sawyer and Sayid the best, but Jack will do in a pinch, " she said. “I used to enlist real people in my, uh, interludes. But then it got too creepy, weird or disappointing, so all my fantasies are agnostic.”

She raises an interesting point, and one echoed by members of the opposite sex. Like my buddy, a former psych student and single man looking for love and lust, who tends not to fantasize with people he mixes with on a daily basis. To him, it would become difficult to deal with those people, as if he’d said something behind their backs.

“In other words, that’s always just seemed like it was objectifying people, whereas the hot woman I’d never known but saw at a dance club or in a restaurant or something isn’t as personal to think of.”

A friend of mine who works in PR and loves the Red Sox said the opposite. He thinks often of the women he is around on a day-to-day basis, along with the ones he wishes he had known intimately.

“I like the girls I fooled around with, but didn’t seal the deal, or went on a few dates and lost touch, or a girl I was too scared to ask out (that hasn’t happened since like high school, though),” he said. “I think there’s more room to make up potential fantasies — places, situations, types of sex, what she’d be wearing — when it’s someone you haven’t been with.”

Whether strangers, friends, past lovers, unrequited lovers, celebrities or porn stars, whom do you fantasize about?