— Richard Feynman American theoretical physicist 1918 - 1988
Does not appear to be from any of his books or cited in a biography. A Google Books search shows that the oldest book citing "physics is like sex" is Scary Monsters and Bright Ideas (2000) by science broadcaster Robyn Williams. On p. 44, this book claims: "Einstein said, 'You do not really understand something unless you can explain it to your grandmother'. Richard Feynman added, 'Physics is like sex: sure, it may give some practical results, but that's not why we do it'." Given that Einstein didn't really say the former, it's likely that Feynman didn't really say the latter.
Disputed and/or attributed
Variante: Physics is like sex: sure, it may give some practical results, but that's not why we do it.