I see this less as a joke and more of a sign of mutual respect...

Feynmann's womanizing was legendary[1]. The fact his wife chose to sue for emotional estrangement instead of explicit acts is interesting. It suggests a residual level of respect or at least a cynical quid pro quo...

[1] See http://tech.mit.edu/V119/N10/col10lipman.10c.html for a somewhat nuanced take on the issue... This was the first link I googled... presumably there are other/better sources...