Referring to the required distance from one's spouse to engage in guilt-free extra-maraital sex and/or to not be considered infidelity.
It is not reccommended to verify spouse's endorsement of such rule but rather follow it discreetly and only reference it as a matter of last resort.
Susie Employee: "Mr. Anderson, I didn't come to this conference to have sex with you, anyway we're both married!
Mr. Anderson: "But Susie, thats the only reason I brought you here and anyway it's okay, it's not cheating because we are well beyond the Fifty-mile rule!"
6👍 1👎