"I cannot tell a lie" joke

A married man and his secretary were having a torrid affair.
One afternoon they couldn't contain their passion, so they
rushed over to her place where they spent the afternoon
making passionate love. When they were finished they fell
asleep and didn't wake up till 8 o'clock.
They got dressed quickly. Then the man told his secretary
to take his shoes outside and rub them on the lawn.
Bewildered, she does as he asks (thinking he's pretty weird).
The man finally gets home and his wife meets him at the door.
Upset, she asks where he's been. The man replies "I cannot
tell a lie. My secretary and I are having an affair. Today
we left work early, went to her place, spent the afternoon
making love then fell asleep. That's why I'm late."
The wife looks at him, takes notice of his shoes and
says, "I see those grass stains on your shoes. You've been
playing golf again, haven't you!?"