I'm watching a trailer for a forthcoming movie called Friends with Benefits with Justin Timberlake and Mila Kunis. Plotwise, it looks exactly like a current Natalie Portman/Ashton Kutcher movie titled No Strings Attached. They're so very alike that I thought, "Huh, they could have saved money and just filmed Mila Kunis having a threesome with Ashton and Justin."

I'd buy a ticket to see that. I won't be heading to the multiplex for these, however. Never watch a movie on a subject about which you are knowledgeable and passionate; it'll just frustrate you. Take any experience—war, parenthood, climbing mountains, being a disabled person, anything—and Hollywood can make a point-missing, reality-ignoring caricature of it. I'm passionate about nontraditional relationships, and just watching these previews makes me want to slap the writers.

The premise of both is this: "We're friends who enjoy spending time together, and we're also going to fuck, frequently and for an ongoing period of time. But we're not going to become emotionally attached beyond casual friendship." And here's a plot spoiler for you: Both movies end with the couple realizing that they're madly and monogamously in love. Wow, crazy twist, right?

I find this cinematic interpretation of a friends-with-benefits arrangement exactly as realistic as a hero leaping from a speeding train onto a moving car or single-handedly solving a murder case that baffled the FBI: not very. How do you conduct a FWB (or NSA) relationship? There's not a single, surefire formula, but it doesn't look just like dating. Some physical distance and infrequency are often built in. Perhaps you get together only at a swing club—one too far away to visit every weekend. Or it's someone you see only at certain social events: your music-festival fuck buddy, your Burning Man bang. In those scenarios, your time together has a clear beginning and end, and it's removed from your regular life, contained in a separate sphere.

But if you regularly and repeatedly have sex with someone you like, you'll become emotionally involved with him or her. And I don't just say that because I'm female. I polled my male swinger/polyamorous pals, and all of them said the movie scenario would lead to emotional attachment for them, too. "I can fuck a friend casually once, maybe twice," one experienced swinger told me. "But more than that, and emotions will be involved."

Those emotions might lead to happily ever after, or they might lead to someone's broken heart. If you're smart and fortunate, you might find a pleasant, sustainable equilibrium of affection and lust that suits you both. But I find it extremely irritating to watch sex scenes in which actors utter lines like "We're not allowed to have any feelings for each other." Feelings don't wait for permission—they just come. You can choose what action to take on them, but you can't simply ignore them if they're inconvenient.

You know what you can ignore? These movies. recommended