“What we know of physical and biological science makes existence of God less probable than the existence of Santa Claus. And the parts of physics that rule out God are not themselves open to much doubt. There is no chance that they will be revised by anything yet to be discovered. To be sure, there will be revolutionary developments in science. Superstring theory may give way to quantum-loop gravity; exceptions to the genetic code may be discovered; some unique function of consciousness may be identified. But there are some things that won't happen. Purposes and designs will never have a role in physics and biology. Perpetual motion machines and other violations of the laws of thermodynamics won't arise, not even if there turns out to be such a thing as cold fusion.”

[The information paradox, arXiv preprint arXiv:hep-th/0612061v2, 14 December 2006, http://arxiv.org/abs/hep-th/0612061] (See also Thorne-Hawking-Preskill bet.)

Source: Living systems, 1978, p. 9-10; As cited in: Kenneth D. Bailey (1994) Sociology and the New Systems Theory: Toward a Theoretical Synthesis. p. 262

Source: The systems view of the world (1996), p. 8 as cited in: Martha C. Beck (2013) "Contemporary Systems Sciences, Implications for the Nature and Value of Religion, the Five Principles of Pancasila, and the Five Pillars of Islam," Dialogue and Universalism-E Volume 4, Number 1/2013. p. 3 ( online http://www.emporia.edu/~cbrown/dnue/documents/vol04.no01.2013/Vol04.01.Beck.pdf).

Price, G.R. (1995). "The nature of selection." Journal of Theoretical Biology 175:389-396 (written circa 1971)

"Loop Quantum Gravity," The New Humanists: Science at the Edge (2003)

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
Variant: Physics is like sex: sure, it may give some practical results, but that's not why we do it.

Source: The Principles of Agriculture, 1844, Section I: The fundamental principles, p. 2.

address http://www.papalencyclicals.net/Pius12/P12EXIST.HTM to the Pontifical Academy of Sciences, 22 November 1951
quoted in Time, 3 December 1951
quoted by Dan Brown, Angels and Demons, page 44

"The Magic of Science" in Imperial Oil Review (Spring, 1994) http://sites.utoronto.ca/jpolanyi/public_affairs/public_affairs4f.html.
Context: It is not the laws of physics that make science possible but the unprovable proposition that there exists a grand design underlying the physical world. And not just any old "grand design" but one that is accessible to the limited senses and modest reasoning powers of the species to which we belong. Scientists subscribe with such conviction to this article of faith that they are willing to commit a lifetime to the pursuit of scientific discovery. It is hardly surprising that an activity so magical is also undefinable. Science is what scientists do. And what they do is look around themselves for messages written in the sky, the earth, the oceans and all living things – messages that tell of the unity of creation. These messages have been there – unseen, though at times written in letters miles high – since the dawn of history. But we have just passed through an epoch in which, quite suddenly, scientists seem to have learnt speed reading. Discoveries have been coming at an unprecedented pace. In the wake of such a period it is common to consider that we may be approaching the point where all that is readable in nature will have been read. We should be skeptical of such claims. Success in reading some messages brings with it a temporary blindness to others. We forget that between the words written in black in nature's book there are likely to be messages of equal importance written in white. It is a truism that success in science comes to the individuals who ask the right questions.