Give it up

An eighty-year-old man was having an annual physical.
As the doctor was listening to his heart with the stethoscope, he began muttering, "Oh oh!"
The man asked the doctor, "What's the problem?"
"Well," said the doc, "you have a serious heart murmur. Do you smoke?"
"No," replied the man.
"Do you drink in excess?"
"No." replied the man.
"Do you have a sex life?"
"Yes, I do!"
"Well," said the doc, "I'm afraid with this heart murmur, you'll have to give up half your sex life."
Looking perplexed, the old man said, "Which half - the looking or the thinking?"