The singer-and-actor has said that being "intimate" with someone without being in a relationship with them is best avoided because it often leads to emotional complications.
He said: "It`s a really good idea until it becomes a bad idea. It probably becomes a bad idea really fast.
"If you`re going to be intimate with someone at some point somebody`s going to feel something."
The 30-year-old heartthrob - who recently split up with Jessica Biel after four years of dating - is currently starring in `Friends With Benefits` with Mila Kunis, which tells the story of two young professionals who decide to have a purely physical relationship which goes wrong when his character falls for Mila`s.
Since breaking up with Jessica he has been romantically linked to the brunette beauty, as well as `Tron` actress Olivia Wilde.
However, talking during a taping of `The Ellen DeGeneres Show`, he maintained that it is impossible for two people to have sex without at least one of them developing some sort of emotion.
TV host Ellen joked: "If it`s bad, then no one`s going to develop any feelings."
Justin retorted: "They`ll develop a feeling but it`s going to be uh, uh."