Unless I am horribly behind the times, people in the US still use 'sleep with' as a euphemism for sex.

This post teaches us all a valuable lesson: always frame the question before, not after, asking it. People stop reading after the '?'

The OP's point was that it might have implied less than that at the time.

No. It's a euphemism for a quantum electrodynamics process, commonly known as pair production. ;)

> Is Feynman using 'sleep with' as a euphemism for sex?

Yes.

I'm pretty sure he's talking about sex.

Yes and no - in those days they had no heating in winter, so people would "sleep with" one another just to keep warm. But it was considered rude to just snuggle up without asking. Of course sometimes this lead to considerable levels of intimacy.