In real life, that's not always the case. Hearts break, emotions flare and sexts get sent long after one party has sworn off the other. Even Kunis and her co-stars aren't fans of the concept.
"It's like communism -- good in theory, in execution it fails," she said in a recent interview with GQ magazine. "Friends of mine have done it, and it never ends well. Why do people put themselves through that torture?"
"For some people, yes," Patricia Clarkson said at the film's New York City premiere, "for me, not so much."
Motivations aside, the experience makes for some entertaining tales. ABCNews.com solicited stories from people who've had not so platonic relationships with their pals. Here, five of the best (or worst, as the case may be):
The Colleague Turned Office Playmate
"I was coming out of a marriage of seven years and decided that a guy I worked with (who I had always had a crush on) and I should do a 'friends with benefits' situation. We 'dated' for four months. Our dating consisted of meeting at my house, motel rooms and talking on the phone. We did hook up a few times at work on a desk. I eventually realized that I wanted more and I deserved more. We decided to just go back to being friends, which was harder than it sounded. I just recently found out that he is now 'dating' someone else we work with. I'm glad I learned my lesson about friends with benefits; it doesn't work." -- Erin.
The Gay Best Friend Turned Friend With Benefits
"It was freshman year of college. After grinding the evening away at Halloween frat party I found myself in my room with a guy who lived in my dorm. We had talked a few times before and I thought he was adorable the moment we met. Excited that we were finally going to hook up, I went in for the kill, only to be instantly pushed back: 'I think I might be gay.' Oh. That was not what I expected to hear.
"I retreated to my corner of the bed. Silence ensued for a solid minute. Panicking to get him to leave, I stuttered, 'Well, if you ever want to talk about it, just let me know.' The evening suddenly became a stock 'Oprah' episode. We actually did keep talking in the days that followed. Initially fueled by the desire to forget the awkwardness of the night, our interactions grew increasingly flirty. Then, one night while watching a movie, his arm slipped around my shoulders. The arm led to hand holding which led to groping which led to making out. Suddenly, my GBF had become my FWB. Things only lasted two weeks between us. While I was willing to look past his sexual questioning, I guess I just wasn't man enough for him." -- Francesca.
The Affair That Blossomed When Mom Wasn't Watching
"It started during a Saturday morning class she attended with her mother, who was pursuing a bachelor of general studies at the same college. We were in the same study group (me, her and her mother) and though her mother was always present, we both seemed to get along fine. One Saturday afternoon after class, her mother had to run an errand, which left the two of us in the library. Conversation quickly took a different turn, since the library was just about empty at 1 p.m. on a summer Saturday afternoon. We talked about what we could be doing. We spent the rest of the afternoon between book aisles doing what friends with benefits do.
"The rest of the semester pretty much revolved around getting early to class, doing our group assignments and once her mother left, it was back to my dorm room, or somewhere in the library. Sometimes I still wonder what became of her, since I later found out she in fact had a boyfriend the entire time. But we agreed to be friends with benefits and nothing more." -- Alani.
The Former FWB Who Still Wants to Sext
"Being the naive little baby I was, I agreed to these terms: We could sleep with together whenever he wanted. And when he wanted to have sex, I obediently obliged. Things went on like that for a while. And by a while, I mean nearly two years. Not even joking. Funny part is: Now that we're apart I'll still get texts from him asking for nude pictures. No dice, buddy." -- Asuka.
The Friend Turned Deadbeat Dad
"I had a best friend/co-worker for several years at our job. I was intrigued by his diverse educational/talented background. He was a teacher/inventor/and musical artist with recorded CDs. He was also a dedicated single dad who doted on his little girl. Everything about him seemed attractive and desirable, and as my abusive marriage began to crumble and my husband and I separated, my relationship with my platonic best friend/co-worker quickly escalated into a steamy hot, romantic, intimate 'friendship' with no strings attached but plenty of benefits!
"For a few years it felt like the 'real' relationship I'd been praying for and lacking in my marriage. Then, in one blissful night filled with passion, adult beverages and broken protection, my best friend gave me one of my most precious treasures. We had a son together although he would have no part in the pregnancy or birth or in the last four years of our sons life. He hems and haws at the idea of a paternity test. I'd say that qualifies as a 'trainwreck.'" -- Nicole.
Submissions have been edited for brevity, last names have been omitted.